feat: checkpoint user state tracking service with PostgreSQL

- RESTful API: POST /heartbeat, POST /checkpoints, GET /status, GET /summaries
- State-change-only checkpoint model with extensible StateType enum
- PostgreSQL backend with sqlx, auto-migration on startup
- pg_cron scheduled aggregation (state_summaries) and offline detection
- Heartbeat-based liveness with 60s timeout auto-offline
- LEAD() window function for state duration calculation
- JSONB content field for extensible checkpoint metadata

BREAKING CHANGE: Complete rewrite from Hello World to full API service.

.gitignore

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