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/nearby.py

@@ -0,0 +1,67 @@
+from fastapi import APIRouter, Depends, Query
+from sqlalchemy.orm import Session
+
+from app.api.deps import lang_param, length_param
+from app.db.session import get_db
+from app.schemas.common import LatLon
+from app.schemas.nearby import NearbyPlace, NearbyResponse
+from app.schemas.place import ContentOut
+from app.services.nearby_search import find_nearby
+
+router = APIRouter(tags=["nearby"])
+
+
+@router.get("/nearby", response_model=NearbyResponse)
+def nearby(
+    lat: float = Query(),
+    lon: float = Query(),
+    city_slug: str | None = Query(default=None),
+    lang: str = Depends(lang_param),
+    length: str = Depends(length_param),
+    min_results: int = Query(default=5, ge=1, le=50),
+    initial_radius_m: int = Query(default=300, ge=50, le=5000),
+    step_m: int = Query(default=200, ge=50, le=5000),
+    max_radius_m: int = Query(default=5000, ge=100, le=20000),
+    speed: float | None = Query(
+        default=None,
+        description="Reserved for v2: user's speed in m/s. Currently accepted but ignored.",
+    ),
+    heading: float | None = Query(
+        default=None,
+        description="Reserved for v2: user's heading in degrees. Currently accepted but ignored.",
+    ),
+    db: Session = Depends(get_db),
+) -> NearbyResponse:
+    result = find_nearby(
+        db,
+        lat=lat,
+        lon=lon,
+        language=lang,
+        length=length,
+        city_slug=city_slug,
+        min_results=min_results,
+        initial_radius_m=initial_radius_m,
+        step_m=step_m,
+        max_radius_m=max_radius_m,
+    )
+
+    places = [
+        NearbyPlace(
+            id=place.id,
+            slug=place.slug,
+            category=place.category,
+            location=LatLon(lat=place_lat, lon=place_lon),
+            address=place.address,
+            district=place.district,
+            built_year=place.built_year,
+            architect_builder=place.architect_builder,
+            architectural_style=place.architectural_style,
+            content=ContentOut(
+                language=content.language, length=content.length, title=content.title, body=content.body
+            ),
+            distance_m=round(distance_m, 1),
+        )
+        for place, content, distance_m, place_lon, place_lat in result.rows
+    ]
+
+    return NearbyResponse(search_radius_m=result.search_radius_m, count=len(places), places=places)