Add Database preferences UI and improve dbfactory robustness

- Add Database tab to GTK preferences UI
- Add connection testing for PostgreSQL
- Lazy import dbpostgres to avoid errors when psycopg2 not installed
- Graceful fallback to SQLite if PostgreSQL connection fails
- Add is_postgres_available() helper function

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Cut5nake