Add FastAPI backend with PostGIS-backed nearby search

FastAPI service backed by PostgreSQL+PostGIS: cities/places/place_content
schema (normalized per-language, per-length content for easy future
re-ingestion), Alembic migration, an iterative-radius-expansion nearby
search endpoint, a name-search fallback for denied-location flows, and
a YAML seed loader CLI. Verified end-to-end against a live Docker
Postgres+PostGIS instance (migration, city seed, place/content CRUD,
and nearby radius expansion all confirmed working via curl).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/api/routes/cities.py

@@ -0,0 +1,34 @@
+from fastapi import APIRouter, Depends
+from geoalchemy2 import Geometry
+from sqlalchemy import cast, func, select
+from sqlalchemy.orm import Session
+
+from app.db.session import get_db
+from app.models.city import City
+from app.schemas.city import CityOut
+from app.schemas.common import LatLon, LocalizedName
+
+router = APIRouter(tags=["cities"])
+
+
+@router.get("/cities", response_model=list[CityOut])
+def list_cities(db: Session = Depends(get_db)) -> list[CityOut]:
+    stmt = select(
+        City,
+        func.ST_X(cast(City.center_point, Geometry)).label("lon"),
+        func.ST_Y(cast(City.center_point, Geometry)).label("lat"),
+    )
+    rows = db.execute(stmt).all()
+
+    out = []
+    for city, lon, lat in rows:
+        center = LatLon(lat=lat, lon=lon) if lat is not None and lon is not None else None
+        out.append(
+            CityOut(
+                id=city.id,
+                slug=city.slug,
+                name=LocalizedName(ru=city.name_ru, en=city.name_en),
+                center=center,
+            )
+        )
+    return out