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/003_sessions.sql

@@ -0,0 +1,65 @@
+-- 003_sessions.sql
+-- 用户会话表 + 心跳管理 + 离线检测 + pg_cron 调度
+
+-- ============================================================
+-- 1. 用户会话表（跟踪当前状态和心跳）
+-- ============================================================
+CREATE TABLE IF NOT EXISTS user_sessions (
+    user_id          VARCHAR(128) PRIMARY KEY,
+    current_state    VARCHAR(64)  NOT NULL DEFAULT 'Offline',
+    last_heartbeat   BIGINT       NOT NULL,
+    last_state_change BIGINT      NOT NULL,
+    updated_at       TIMESTAMPTZ  NOT NULL DEFAULT now()
+);
+
+-- ============================================================
+-- 2. 离线检测函数：心跳超时 60s 的用户自动补一条 Offline 检查点
+--    pg_cron 每 1 分钟执行一次
+-- ============================================================
+CREATE OR REPLACE FUNCTION detect_offline_users(
+    timeout_secs BIGINT DEFAULT 60
+) RETURNS SETOF BIGINT AS $$
+DECLARE
+    r RECORD;
+    now_ts BIGINT;
+BEGIN
+    now_ts := EXTRACT(EPOCH FROM now())::BIGINT;
+
+    FOR r IN
+        SELECT user_id, last_heartbeat
+        FROM user_sessions
+        WHERE current_state != 'Offline'
+          AND (now_ts - last_heartbeat) > timeout_secs
+    LOOP
+        -- 插入离线检查点（时间戳 = 最后心跳 + 超时时间）
+        INSERT INTO checkpoints (user_id, state, timestamp)
+        VALUES (r.user_id, 'Offline', r.last_heartbeat + timeout_secs);
+
+        -- 更新会话状态
+        UPDATE user_sessions
+        SET current_state = 'Offline',
+            last_state_change = r.last_heartbeat + timeout_secs,
+            updated_at = now()
+        WHERE user_id = r.user_id;
+
+        RETURN NEXT 1;
+    END LOOP;
+END;
+$$ LANGUAGE plpgsql;
+
+-- ============================================================
+-- 3. pg_cron 调度（需要超级用户手动执行一次）
+-- ============================================================
+-- 
+-- -- 安装扩展（仅需一次）
+-- CREATE EXTENSION IF NOT EXISTS pg_cron;
+--
+-- -- 每 1 分钟检测离线用户
+-- SELECT cron.schedule(
+--     'detect-offline',
+--     '* * * * *',
+--     $$ SELECT detect_offline_users(60); $$
+-- );
+--
+-- -- 查看状态
+-- SELECT * FROM cron.job;