Scaffold async database connection

[podmar]

Covers async DB connection layer.

Changes

• app/database.py — async SQLAlchemy engine, session factory, and create_db_and_tables() startup hook
• app/main.py — wired lifespan context manager to call create_db_and_tables() on startup
• pyproject.toml — added sqlmodel, asyncpg, python-dotenv

Why these packages

• SQLModel unifies SQLAlchemy (DB layer) and Pydantic (API layer) into one model class, avoiding duplicate schema definitions
• asyncpg is the native async Postgres driver — the standard psycopg2 would block the event loop on every query
• python-dotenv loads .env locally; on hosted platforms the env var comes from the platform directly and dotenv silently no-ops

Key patterns

• One engine per process (connection pool, not per-request connections)
• get_session as a FastAPI Depends — opens before the handler, guaranteed close/rollback after
• expire_on_commit=False — keeps objects readable post-commit without a follow-up query (required in async, where lazy loading isn't available)
• lifespan over the deprecated @app.on_event("startup")