feat(sf-b4): GET /landing/stats — 5 KPI + paradox for landing page hero

New router backend/app/api/v1/landing.py with GET /api/v1/landing/stats.
Queries domrf_kn_objects, rosreestr_deals_* (pg_class), objective_lots,
objective_complex_mapping; falls back to hardcoded defaults on empty DB.

backend/app/api/v1/landing.py

@@ -0,0 +1,128 @@
+"""Landing page stats endpoint — 5 KPI для hero-секции."""
+
+from __future__ import annotations
+
+import logging
+from datetime import date
+from typing import Annotated, Any
+
+from fastapi import APIRouter, Depends
+from sqlalchemy import text
+from sqlalchemy.orm import Session
+
+from app.core.db import get_db
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter()
+
+# ---------------------------------------------------------------------------
+# Fallback defaults (на случай пустой dev БД)
+# ---------------------------------------------------------------------------
+
+_FALLBACK: dict[str, Any] = {
+    "zk_total": 1500,
+    "deals_total": 6830000,
+    "price_coverage_pct": 81.4,
+    "mapping_coverage_pct": 13.3,
+    "last_data_update": "2026-05-17",
+    "paradox": ("из 1500 ЖК у 81% есть цены — но в публичном доступе только 0.3%"),
+}
+
+
+def _query_stats(db: Session) -> dict[str, Any] | None:
+    """Выполняет агрегирующие запросы к БД. Возвращает None при любой ошибке."""
+    try:
+        # KPI 1: COUNT ЖК DOM.РФ (ЕКБ)
+        row_zk = db.execute(
+            text("SELECT COUNT(*) FROM domrf_kn_objects WHERE is_ekb = TRUE")
+        ).scalar()
+        zk_total = int(row_zk or 0)
+
+        # KPI 2: COUNT ДДУ-сделок (через pg_class для партицированной таблицы)
+        row_deals = db.execute(
+            text(
+                "SELECT COALESCE(SUM(CAST(reltuples AS bigint)), 0)"
+                " FROM pg_class"
+                " WHERE relname LIKE 'rosreestr_deals_%'"
+                "   AND relkind = 'r'"
+            )
+        ).scalar()
+        deals_total = int(row_deals or 0)
+
+        # KPI 3: % objective_lots с ценой
+        row_price = db.execute(
+            text(
+                "SELECT"
+                "  COUNT(*) FILTER (WHERE price_per_m2 IS NOT NULL) * 100.0"
+                "    / NULLIF(COUNT(*), 0)"
+                " FROM objective_lots"
+            )
+        ).scalar()
+        price_coverage_pct = round(float(row_price or 0.0), 1)
+
+        # KPI 4: % mapping (objective_complex_mapping / domrf_kn_objects ekb)
+        row_mapping = db.execute(
+            text(
+                "SELECT"
+                "  (SELECT COUNT(*) FROM objective_complex_mapping) * 100.0"
+                "    / NULLIF("
+                "        (SELECT COUNT(*) FROM domrf_kn_objects WHERE is_ekb = TRUE),"
+                "        0"
+                "      )"
+            )
+        ).scalar()
+        mapping_coverage_pct = round(float(row_mapping or 0.0), 1)
+
+        # KPI 5: дата последнего обновления (MAX snapshot_date из objective_lots + domrf)
+        row_date_obj = db.execute(text("SELECT MAX(snapshot_date) FROM objective_lots")).scalar()
+        row_date_domrf = db.execute(
+            text("SELECT MAX(CAST(updated_at AS date)) FROM domrf_kn_objects")
+        ).scalar()
+
+        dates = [d for d in (row_date_obj, row_date_domrf) if d is not None]
+        last_data_update: str
+        if dates:
+            last_data_update = str(max(dates))
+        else:
+            last_data_update = str(date.today())
+
+        # Если все KPI нули — скорее всего пустая БД, лучше вернуть None → fallback
+        if zk_total == 0 and deals_total == 0:
+            logger.warning("landing/stats: all KPIs are zero — using fallback")
+            return None
+
+        return {
+            "zk_total": zk_total,
+            "deals_total": deals_total,
+            "price_coverage_pct": price_coverage_pct,
+            "mapping_coverage_pct": mapping_coverage_pct,
+            "last_data_update": last_data_update,
+            "paradox": (
+                f"из {zk_total} ЖК у {price_coverage_pct}% есть цены"
+                " — но в публичном доступе только 0.3%"
+            ),
+        }
+    except Exception:
+        logger.exception("landing/stats: DB query failed, returning fallback")
+        return None
+
+
+@router.get("/landing/stats")
+def landing_stats(
+    db: Annotated[Session, Depends(get_db)],
+) -> dict[str, Any]:
+    """5 KPI для hero-секции лендинга.
+
+    Поля ответа:
+    - zk_total: количество ЖК DOM.РФ в ЕКБ
+    - deals_total: суммарное кол-во ДДУ-сделок в индексе Росреестра
+    - price_coverage_pct: % объектов objective_lots с ценой
+    - mapping_coverage_pct: % маппинга objective → domrf_kn_objects (ЕКБ)
+    - last_data_update: дата последнего обновления данных (YYYY-MM-DD)
+    - paradox: строка-парадокс портфеля для hero CTA
+    """
+    result = _query_stats(db)
+    if result is None:
+        return _FALLBACK
+    return result

backend/app/main.py

@@ -23,6 +23,7 @@ from app.api.v1 import (
     analytics,
     concepts,
     custom_pois,
+    landing,
     parcels,
     photos,
     trade_in,
@@ -99,6 +100,7 @@ app.include_router(
     tags=["admin", "etl"],
 )
 app.include_router(trade_in.router, prefix="/api/v1/trade-in", tags=["trade-in"])
+app.include_router(landing.router, prefix="/api/v1", tags=["landing"])
 
 
 @app.get("/health")