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/services/geocoding.py

@@ -0,0 +1,47 @@
+"""Fallback name/address search over our own `places` dataset.
+
+Used when the user denies location permission and types a place of interest,
+or when 2GIS's own geocoding API isn't wired into a given client flow yet.
+"""
+
+from geoalchemy2 import Geometry
+from sqlalchemy import and_, cast, func, select
+from sqlalchemy.orm import Session
+
+from app.models.city import City
+from app.models.enums import ContentLanguage, ContentLength
+from app.models.place import Place
+from app.models.place_content import PlaceContent
+
+
+def search_places(
+    db: Session,
+    *,
+    q: str,
+    language: ContentLanguage,
+    city_slug: str | None = None,
+    limit: int = 20,
+):
+    stmt = (
+        select(
+            Place,
+            PlaceContent,
+            func.ST_X(cast(Place.location, Geometry)).label("lon"),
+            func.ST_Y(cast(Place.location, Geometry)).label("lat"),
+        )
+        .join(
+            PlaceContent,
+            and_(
+                PlaceContent.place_id == Place.id,
+                PlaceContent.language == language,
+                PlaceContent.length == ContentLength.short,
+            ),
+        )
+        .where(Place.is_active.is_(True))
+        .where(PlaceContent.title.ilike(f"%{q}%"))
+        .limit(limit)
+    )
+    if city_slug is not None:
+        stmt = stmt.join(City, City.id == Place.city_id).where(City.slug == city_slug)
+
+    return db.execute(stmt).all()