backend: fix mypy errors in geometry stub

- empty_buildings: dict → dict[str, Any] (mypy strict needs type args)
- drop unused `# type: ignore[arg-type]` on strategy= (mypy now narrows
  the loop variable to Literal[...] from a tuple of literals)

backend/app/services/generative/geometry.py

@@ -5,12 +5,14 @@ Stage 1b: greedy filling of rectangular MKD with 3 strategies. STRtree for colli
 Performance target: <=10s per variant; fallback acceptance 15s.
 """
 
+from typing import Any
+
 from app.schemas.concept import TEAP, ConceptInput, ConceptVariant, FinancialModel
 
 
 def generate_stub(payload: ConceptInput) -> list[ConceptVariant]:
     """Placeholder returning 3 empty variants. Replaced in Stage 1b."""
-    empty_buildings: dict = {"type": "FeatureCollection", "features": []}
+    empty_buildings: dict[str, Any] = {"type": "FeatureCollection", "features": []}
     empty_teap = TEAP(
         built_area_sqm=0.0,
         total_floor_area_sqm=0.0,
@@ -26,7 +28,7 @@ def generate_stub(payload: ConceptInput) -> list[ConceptVariant]:
     for strategy in ("max_area", "max_insolation", "balanced"):
         strategies.append(
             ConceptVariant(
-                strategy=strategy,  # type: ignore[arg-type]
+                strategy=strategy,
                 buildings_geojson=empty_buildings,
                 teap=empty_teap,
                 financial=empty_financial,