diff --git a/backend/scripts/spike_plan_vectorize.py b/backend/scripts/spike_plan_vectorize.py
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+++ b/backend/scripts/spike_plan_vectorize.py
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+"""Spike — contour-vectorize floor-plan rasters (PNG/JPG → SVG) via Potrace.
+
+Forgejo issue #299 (Phase 1 exploration SPIKE). **This is a throwaway probe, NOT
+production wiring**: it answers one question — is Potrace contour tracing good
+enough to turn floor-plan rasters into compact, readable SVG for the planning UI
+catalog and PDF reports? No ML, no room semantics, no DOM.RF coupling.
+
+WHAT IT MEASURES
+----------------
+For each input raster the pipeline runs:
+
+  1. load (Pillow) → grayscale (``L``)
+  2. binarise: pixels below ``--threshold`` (0-255) become ink (black), the rest
+     become paper (white). Optional ``--invert`` for light-on-dark plans.
+  3. write a 1-bit PBM bitmap (Potrace's native input) to the output dir
+  4. ``potrace -s`` → SVG (centerline-free, filled-contour tracing)
+  5. record size metrics: raster bytes, svg bytes, ratio (raster / svg)
+
+It also (optionally, ``--render-back``) rasterises the produced SVG back to PNG
+via ``rsvg-convert`` so a human can eyeball SVG-vs-original side by side and
+judge whether wall/room contours survived the round trip.
+
+WHY POTRACE IS SOURCE-AGNOSTIC
+------------------------------
+Potrace traces the boundary between ink and paper regions of a bitmap. It does
+not care where the raster came from — a Wikimedia line-drawing plan, a scanned
+blueprint, or a DOM.RF UI tile all reduce to "dark strokes on a light field"
+after thresholding. So the Phase 1 question ("are the contours traceable and how
+much do they compress?") is validly answered on any representative floor-plan
+rasters; see the spike doc for the explicit DOM.RF caveat.
+
+USAGE
+-----
+    # vectorise every raster in a folder, collect a metrics table
+    uv run python backend/scripts/spike_plan_vectorize.py \
+        --in-dir /tmp/plan-spike/in --out-dir /tmp/plan-spike/out
+
+    # also render the SVGs back to PNG for visual QA
+    uv run python backend/scripts/spike_plan_vectorize.py \
+        --in-dir /tmp/plan-spike/in --out-dir /tmp/plan-spike/out --render-back
+
+Requires the ``potrace`` binary on PATH (``brew install potrace``) and, for
+``--render-back``, ``rsvg-convert`` (``brew install librsvg``). Pillow ships with
+the backend deps.
+"""
+
+from __future__ import annotations
+
+import argparse
+import logging
+import shutil
+import statistics
+import subprocess
+import sys
+from dataclasses import dataclass
+from pathlib import Path
+
+from PIL import Image
+
+logger = logging.getLogger("spike_plan_vectorize")
+
+_RASTER_SUFFIXES = {".png", ".jpg", ".jpeg", ".bmp", ".gif", ".tif", ".tiff"}
+_POTRACE_TIMEOUT_S = 120
+_RSVG_TIMEOUT_S = 120
+
+
+@dataclass(frozen=True)
+class VectorizeResult:
+    """One raster's trip through the pipeline."""
+
+    name: str
+    raster_bytes: int
+    svg_bytes: int
+    svg_path: Path
+    rendered_path: Path | None
+
+    @property
+    def ratio(self) -> float:
+        """Compression ratio raster / svg (>1 means SVG is smaller)."""
+        return self.raster_bytes / self.svg_bytes if self.svg_bytes else 0.0
+
+    def metrics_line(self) -> str:
+        return (
+            f"{self.name:<44} "
+            f"raster={self.raster_bytes:>9}B  "
+            f"svg={self.svg_bytes:>8}B  "
+            f"ratio={self.ratio:>6.2f}x"
+        )
+
+
+def _require_binary(name: str) -> str:
+    """Resolve an external binary on PATH or exit with a clear message."""
+    path = shutil.which(name)
+    if path is None:
+        logger.error("required binary %r not found on PATH", name)
+        raise SystemExit(
+            f"{name!r} not found — install it (e.g. `brew install {name}`) and retry"
+        )
+    return path
+
+
+def raster_to_pbm(src: Path, pbm_path: Path, *, threshold: int, invert: bool) -> None:
+    """Load a raster, grayscale + threshold it, and write a 1-bit PBM bitmap.
+
+    Potrace consumes 1-bit bitmaps (PBM/PGM/BMP). We binarise with a fixed
+    threshold so the spike's behaviour is deterministic and explainable — black
+    (ink) is what Potrace traces, white is background. ``invert`` flips the test
+    for light-stroke-on-dark plans.
+    """
+    with Image.open(src) as im:
+        gray = im.convert("L")
+    # point(): pixel < threshold → ink (0), else paper (255). Pillow's "1" mode
+    # then packs to a true 1-bit bitmap that Potrace reads natively.
+    cutoff = threshold
+    if invert:
+        bitmap = gray.point(lambda px: 0 if px >= cutoff else 255).convert("1")
+    else:
+        bitmap = gray.point(lambda px: 0 if px < cutoff else 255).convert("1")
+    bitmap.save(pbm_path)
+
+
+def pbm_to_svg(potrace_bin: str, pbm_path: Path, svg_path: Path) -> None:
+    """Trace a PBM bitmap to SVG with ``potrace -s`` (SVG backend)."""
+    try:
+        subprocess.run(
+            [potrace_bin, "-s", "-o", str(svg_path), str(pbm_path)],
+            check=True,
+            capture_output=True,
+            timeout=_POTRACE_TIMEOUT_S,
+        )
+    except subprocess.CalledProcessError as exc:
+        stderr = exc.stderr.decode("utf-8", "replace").strip()
+        logger.error("potrace failed for %s: %s", pbm_path.name, stderr)
+        raise
+
+
+def svg_to_png(rsvg_bin: str, svg_path: Path, png_path: Path, *, width: int) -> None:
+    """Render an SVG back to PNG via ``rsvg-convert`` for visual QA.
+
+    We composite onto an explicit white background. Potrace's SVG is black
+    filled paths on a *transparent* canvas; without ``--background-color=white``
+    rsvg renders the black fill onto transparency and a flattening viewer shows
+    a solid black tile. White matches the real catalog/PDF use case anyway.
+    """
+    try:
+        subprocess.run(
+            [
+                rsvg_bin, "-w", str(width),
+                "--background-color=white",
+                "-o", str(png_path), str(svg_path),
+            ],
+            check=True,
+            capture_output=True,
+            timeout=_RSVG_TIMEOUT_S,
+        )
+    except subprocess.CalledProcessError as exc:
+        stderr = exc.stderr.decode("utf-8", "replace").strip()
+        logger.error("rsvg-convert failed for %s: %s", svg_path.name, stderr)
+        raise
+
+
+def vectorize_one(
+    src: Path,
+    out_dir: Path,
+    *,
+    potrace_bin: str,
+    rsvg_bin: str | None,
+    threshold: int,
+    invert: bool,
+    render_width: int,
+) -> VectorizeResult:
+    """Run the full PNG/JPG → PBM → SVG (→ PNG) pipeline for a single raster."""
+    stem = src.stem
+    pbm_path = out_dir / f"{stem}.pbm"
+    svg_path = out_dir / f"{stem}.svg"
+
+    raster_to_pbm(src, pbm_path, threshold=threshold, invert=invert)
+    pbm_to_svg(potrace_bin, pbm_path, svg_path)
+
+    rendered_path: Path | None = None
+    if rsvg_bin is not None:
+        rendered_path = out_dir / f"{stem}.rendered.png"
+        svg_to_png(rsvg_bin, svg_path, rendered_path, width=render_width)
+
+    return VectorizeResult(
+        name=src.name,
+        raster_bytes=src.stat().st_size,
+        svg_bytes=svg_path.stat().st_size,
+        svg_path=svg_path,
+        rendered_path=rendered_path,
+    )
+
+
+def discover_rasters(in_dir: Path) -> list[Path]:
+    """Return sorted raster files in ``in_dir`` (non-recursive)."""
+    return sorted(
+        p for p in in_dir.iterdir() if p.is_file() and p.suffix.lower() in _RASTER_SUFFIXES
+    )
+
+
+def summarise(results: list[VectorizeResult]) -> None:
+    """Print a per-file metrics table plus min/median/max compression."""
+    if not results:
+        logger.warning("no results to summarise")
+        return
+
+    print("\n=== per-raster metrics ===")
+    for r in results:
+        print(r.metrics_line())
+
+    ratios = sorted(r.ratio for r in results)
+    print("\n=== compression summary ===")
+    print(f"samples       : {len(ratios)}")
+    print(f"min ratio     : {min(ratios):.2f}x")
+    print(f"median ratio  : {statistics.median(ratios):.2f}x")
+    print(f"max ratio     : {max(ratios):.2f}x")
+    total_raster = sum(r.raster_bytes for r in results)
+    total_svg = sum(r.svg_bytes for r in results)
+    print(f"aggregate     : {total_raster}B raster -> {total_svg}B svg "
+          f"({total_raster / total_svg:.2f}x overall)")
+
+
+def build_parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        description="Spike: contour-vectorize floor-plan rasters (PNG/JPG → SVG) via Potrace.",
+    )
+    parser.add_argument(
+        "--in-dir", type=Path, required=True,
+        help="folder containing input rasters (PNG/JPG/...)",
+    )
+    parser.add_argument(
+        "--out-dir", type=Path, required=True,
+        help="folder for output .pbm/.svg (and .rendered.png with --render-back)",
+    )
+    parser.add_argument(
+        "--threshold", type=int, default=128,
+        help="grayscale ink/paper cutoff 0-255 (default 128)",
+    )
+    parser.add_argument(
+        "--invert", action="store_true",
+        help="treat light strokes on a dark field as ink",
+    )
+    parser.add_argument(
+        "--render-back", action="store_true",
+        help="rasterise each SVG back to PNG via rsvg-convert for visual QA",
+    )
+    parser.add_argument(
+        "--render-width", type=int, default=900,
+        help="width in px for --render-back output (default 900)",
+    )
+    return parser
+
+
+def main(argv: list[str] | None = None) -> int:
+    logging.basicConfig(level=logging.INFO, format="%(levelname)s %(name)s: %(message)s")
+    args = build_parser().parse_args(argv)
+
+    in_dir: Path = args.in_dir
+    out_dir: Path = args.out_dir
+    if not in_dir.is_dir():
+        logger.error("input dir does not exist: %s", in_dir)
+        return 2
+    if not 0 <= args.threshold <= 255:
+        logger.error("--threshold must be 0-255, got %d", args.threshold)
+        return 2
+    out_dir.mkdir(parents=True, exist_ok=True)
+
+    potrace_bin = _require_binary("potrace")
+    rsvg_bin = _require_binary("rsvg-convert") if args.render_back else None
+
+    rasters = discover_rasters(in_dir)
+    if not rasters:
+        logger.error("no raster files found in %s", in_dir)
+        return 1
+    logger.info("found %d raster(s) in %s", len(rasters), in_dir)
+
+    results: list[VectorizeResult] = []
+    failed = 0
+    for src in rasters:
+        try:
+            results.append(
+                vectorize_one(
+                    src,
+                    out_dir,
+                    potrace_bin=potrace_bin,
+                    rsvg_bin=rsvg_bin,
+                    threshold=args.threshold,
+                    invert=args.invert,
+                    render_width=args.render_width,
+                )
+            )
+            logger.info("vectorized %s", src.name)
+        except Exception:
+            # Log + continue: one bad raster must not abort the batch, but we
+            # never swallow silently — the traceback is recorded and the file
+            # is counted as a failure in the final tally.
+            failed += 1
+            logger.exception("failed to vectorize %s", src.name)
+
+    summarise(results)
+    if failed:
+        logger.warning("%d/%d raster(s) failed", failed, len(rasters))
+    return 0 if results else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())