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/models/enums.py

@@ -0,0 +1,25 @@
+import enum
+
+
+class PlaceCategory(str, enum.Enum):
+    building = "building"
+    bridge = "bridge"
+    monument = "monument"
+    park = "park"
+    district = "district"
+    skyscraper = "skyscraper"
+    cathedral = "cathedral"
+    museum = "museum"
+    gate = "gate"
+    square = "square"
+
+
+class ContentLanguage(str, enum.Enum):
+    ru = "ru"
+    en = "en"
+
+
+class ContentLength(str, enum.Enum):
+    short = "short"
+    medium = "medium"
+    long = "long"