feat(sync-history): 新增同步结果查询(每次自动/手动/立即同步的记录)

后端：
- db.py SCHEMA 新增 sync_history 表(不可变,每次同步尝试一行) + 索引
- garmin.py: _log_sync_history() 在 sync_data 全部出口记录; trigger 区分
  auto(调度器)/manual(同步页开始同步)/quick(设置页立即同步); start_sync
  限流拦截分支同样留档; 写入失败只告警不影响同步
- scheduler.py 自动同步传 trigger="auto"; routes 新增 GET /garmin/sync-history
  按时间倒序返回(默认 50 条,上限 200)

前端：
- api.ts 增加 SyncHistoryItem 类型 + getSyncHistory()
- 新页面 /sync-history/ 同步记录: 卡片列表, 状态(成功/失败/被限流)chip 配色,
  时间本地化(今天/昨天/X月X日), 触发类型标签, 范围与耗时
- 同步页与设置页同步区块均加入口链接

backend/db.py

@@ -87,6 +87,31 @@ CREATE TABLE IF NOT EXISTS sync_status (
   FOREIGN KEY (user_id) REFERENCES users(id)
 );
 
+-- One immutable row per sync attempt (auto / manual / 立即同步), so the user
+-- can see what ran and what happened. sync_status above only keeps the
+-- *current* state; it says nothing about a run that finished hours ago.
+-- Column is trigger_kind, not trigger: TRIGGER is a MariaDB reserved word and
+-- a bare `trigger` column fails with 1064 there (SQLite tolerates it, so a
+-- local green suite does not catch it).
+CREATE TABLE IF NOT EXISTS sync_history (
+  id VARCHAR(64) PRIMARY KEY,
+  user_id VARCHAR(64) NOT NULL,
+  trigger_kind VARCHAR(16) NOT NULL,
+  status VARCHAR(24) NOT NULL,
+  days INT DEFAULT 0,
+  records_synced INT DEFAULT 0,
+  activities_synced INT DEFAULT 0,
+  badges_synced INT DEFAULT 0,
+  personal_records_synced INT DEFAULT 0,
+  started_at DATETIME NOT NULL,
+  finished_at DATETIME,
+  duration_seconds INT DEFAULT 0,
+  message TEXT,
+  FOREIGN KEY (user_id) REFERENCES users(id)
+);
+CREATE INDEX IF NOT EXISTS idx_sync_history_user_time
+  ON sync_history (user_id, started_at);
+
 -- Garmin OAuth tokens, obtained once through an interactive login.
 -- Garmin accounts with two-factor auth cannot be logged into unattended: the
 -- library asks for an MFA code on stdin, which a gunicorn worker does not