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.

migrations/001_init.sql

@@ -0,0 +1,20 @@
+-- 001_init.sql
+-- Checkpoint 服务初始建表
+
+CREATE TABLE IF NOT EXISTS checkpoints (
+    id         BIGSERIAL PRIMARY KEY,
+    user_id    VARCHAR(128) NOT NULL,
+    state      VARCHAR(64)  NOT NULL,
+    timestamp  BIGINT       NOT NULL,
+    content    JSONB,
+
+    created_at TIMESTAMPTZ  NOT NULL DEFAULT now()
+);
+
+-- 核心查询索引：按用户 + 时间范围
+CREATE INDEX IF NOT EXISTS idx_checkpoints_user_ts
+    ON checkpoints (user_id, timestamp);
+
+-- 快速获取用户最新检查点
+CREATE INDEX IF NOT EXISTS idx_checkpoints_user_latest
+    ON checkpoints (user_id, timestamp DESC);