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/city.py

@@ -0,0 +1,23 @@
+from datetime import datetime
+
+from geoalchemy2 import Geography
+from sqlalchemy import DateTime, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base import Base
+
+
+class City(Base):
+    __tablename__ = "cities"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    slug: Mapped[str] = mapped_column(String(64), unique=True, nullable=False)
+    name_ru: Mapped[str] = mapped_column(String(128), nullable=False)
+    name_en: Mapped[str] = mapped_column(String(128), nullable=False)
+    country_code: Mapped[str] = mapped_column(String(2), nullable=False, default="RU")
+    center_point = mapped_column(
+        Geography(geometry_type="POINT", srid=4326, spatial_index=False), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+
+    places: Mapped[list["Place"]] = relationship(back_populates="city", cascade="all, delete-orphan")