[db] Phase 5: PostgreSQL support (opt-in over SQLite)

[mfittko]

Summary

Enable PostgreSQL as an optional database backend (keeping SQLite as default). Provide configuration switches, Docker Compose wiring, and documentation. Tests for both backends.

Docs: docs/issues/phase-5-postgres-support.md

Motivation

• SQLite is great for dev; Postgres is preferred in production
• Provide a simple switch to use managed Postgres

Scope

• Config: DB_DRIVER=sqlite|postgres (default sqlite), DATABASE_URL for Postgres
• App driver selection and bootstrap/migrations
• Docker Compose: local Postgres service with healthcheck + volume
• README/docs for DB setup and quick start
• Tests (unit + minimal integration)
• Optional CI job to run Postgres integration tests

Acceptance Criteria

• App runs with SQLite by default
• App runs with Postgres when configured; migrations applied
• Compose Postgres works out-of-the-box for local dev
• Documentation updated

References

• docs/issues/phase-5-postgres-support.md
• docs/issues/phase-7-db-optimization.md (optimization and tuning follow-up)