1. Project Setup & Database Infrastructure

## Parent PRD

#1

## What to build

Establish the foundation for the entire backend system. Set up FastAPI project structure with dependency injection, PostgreSQL with PostGIS for geospatial queries, database migrations, Docker containerization, and environment configuration. This slice enables all subsequent slices by providing:
- FastAPI application with dependency injection framework
- Database connection pooling and health checks
- Alembic migration system for schema management
- PostGIS extension setup for location-based queries
- Docker/docker-compose for local development and deployment
- Environment variable configuration and secrets management

This is pure infrastructure with no business logic; it's the foundation all other slices depend on.

## Acceptance criteria

- [ ] FastAPI app boots with dependency injection container
- [ ] PostgreSQL database connects and PostGIS extension is installed
- [ ] Alembic migrations can create/rollback schema changes
- [ ] Docker image builds and docker-compose runs backend + postgres
- [ ] Environment config loads from .env (DATABASE_URL, SECRET_KEY, etc.)
- [ ] Health check endpoint `GET /health` returns 200 OK
- [ ] Database connection pooling is configured (min_size=5, max_size=20)

## Blocked by

None - can start immediately

## User stories addressed

This is infrastructure supporting all user stories.