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.

Cargo.toml

@@ -4,3 +4,12 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+axum = "0.8"
+tokio = { version = "1", features = ["full"] }
+tower-http = { version = "0.6", features = ["cors"] }
+chrono = "0.4.44"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+async-trait = "0.1"
+dotenvy = "0.15"
+sqlx = { version = "0.8", features = ["runtime-tokio", "postgres", "chrono"] }