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/schemas/place.py

@@ -0,0 +1,32 @@
+from pydantic import BaseModel
+
+from app.models.enums import ContentLanguage, ContentLength, PlaceCategory
+from app.schemas.common import LatLon
+
+
+class ContentOut(BaseModel):
+    language: ContentLanguage
+    length: ContentLength
+    title: str
+    body: str
+
+
+class PlaceListItem(BaseModel):
+    id: int
+    slug: str
+    category: PlaceCategory
+    name: str
+    location: LatLon
+
+
+class PlaceDetail(BaseModel):
+    id: int
+    slug: str
+    category: PlaceCategory
+    location: LatLon
+    address: str | None = None
+    district: str | None = None
+    built_year: str | None = None
+    architect_builder: str | None = None
+    architectural_style: str | None = None
+    content: ContentOut