feat: PostgreSQL support via DATABASE_URL env variable

[cokehill]

Summary

SQLite is perfect for self-hosted single-instance deployments, but teams deploying to
Railway/Render/Fly.io with horizontal scaling need PostgreSQL.

Acceptance criteria

• If DATABASE_URL env var is set, use pg (node-postgres) instead of better-sqlite3
• DB abstraction layer in src/db/database.js detects which driver to use
• Migrations run on both SQLite and Postgres (use raw SQL, not an ORM)
• All queries work unchanged on both backends
• docker-compose.yml gains an optional postgres service profile
• README documents DATABASE_URL format

Notes

Keep better-sqlite3 as the default — SQLite requires zero config.
Consider a lightweight adapter interface: { prepare, exec } for SQLite vs { query } for Postgres.
Only add pg as a dependency (not pg-native).