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>

docker-compose.yml

@@ -0,0 +1,39 @@
+services:
+  db:
+    image: postgis/postgis:16-3.4
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+    ports:
+      - "5432:5432"
+    volumes:
+      - guidecity_pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+  api:
+    build: ./backend
+    environment:
+      DATABASE_URL: postgresql+psycopg://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}
+    ports:
+      - "8000:8000"
+    depends_on:
+      db:
+        condition: service_healthy
+    volumes:
+      - ./backend:/app
+    command: uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload
+
+  adminer:
+    image: adminer
+    ports:
+      - "8080:8080"
+    depends_on:
+      - db
+
+volumes:
+  guidecity_pgdata: