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/tests/test_nearby.py

@@ -0,0 +1,25 @@
+"""Integration test — assumes the DB has been migrated and seeded
+(see README: alembic upgrade head + seed_loader for all 3 place files)
+before running `docker compose exec api pytest`.
+"""
+
+from fastapi.testclient import TestClient
+
+from app.main import app
+
+client = TestClient(app)
+
+
+def test_nearby_red_square_returns_at_least_five_places() -> None:
+    response = client.get(
+        "/api/v1/nearby",
+        params={"lat": 55.7539, "lon": 37.6208, "city_slug": "moscow", "lang": "ru", "length": "short"},
+    )
+    assert response.status_code == 200
+
+    body = response.json()
+    assert body["count"] >= 5
+    assert len(body["places"]) == body["count"]
+
+    distances = [p["distance_m"] for p in body["places"]]
+    assert distances == sorted(distances)