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/Dockerfile

@@ -0,0 +1,16 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libpq-dev gcc \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt requirements-dev.txt ./
+RUN pip install --no-cache-dir -r requirements-dev.txt
+
+COPY . .
+
+EXPOSE 8000
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]