refactor(sync): 手动同步与自动同步彻底分开

「历史范围」原本放在设置页，却只对同步页的一个按钮起作用；而同步页最
显眼的主按钮「同步最新数据」写死 2 天，根本不看这个设置。选了「全部
历史」再点主按钮，表现就是应用无视你 —— 这正是反复出现的「只同步下来
两天」。

现在两条链路各管各的：

* 自动同步：只在设置页配置（开关 + 频率），窗口固定 SYNC_DAYS，不再
  读 history_days。措辞也改成「拉取最近几天」，不再暗示会补历史。
* 手动同步：范围就在同步页当场选，紧挨着用它的按钮，并标出每个范围的
  实际代价（自上次同步 / 7 天 / … / 全部历史约 730 天、20-40 分钟）。
  两个按钮合成一个「开始同步」，写死 2 天的那个删掉。

history_days 保留为「上次手动选的范围」，只有同步页读它；默认值改成
-1（自上次同步），对日常使用是正确的起点。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/services/scheduler.py

@@ -128,10 +128,11 @@ def sync_all_accounts(days=None, respect_schedule=False):
     that have auto-sync off or that were synced recently enough. A direct call
     (a manual "sync everything") leaves it False and syncs unconditionally.
 
-    When `days` is not provided, each account's saved `history_days` from
-    `user_settings` is used (the user's 历史范围 picker), falling back to
-    `SYNC_DAYS`. The scheduled loop always passes `days` explicitly — 历史范围
-    is the window for the manual 全量同步, not for a half-hourly tick.
+    The window is `SYNC_DAYS` unless the caller overrides it. Auto-sync is
+    deliberately blind to the user's manual sync range: reading it here is what
+    let a half-hourly tick re-pull 730 days and hold the account in a 429 loop,
+    and it also made one setting mean two different things in two places.
+    Anything the user chooses by hand goes through `/garmin/sync` instead.
     """
     rows = query_all("SELECT user_id FROM garmin_tokens")
     results = []
@@ -148,22 +149,7 @@ def sync_all_accounts(days=None, respect_schedule=False):
                     results.append({"user": uid, "status": "skipped",
                                     "reason": "not due"})
                     continue
-            # Resolve the sync window: prefer the user's saved history_days,
-            # then the caller override, then the global default.
-            if days is None:
-                # query_one returns a dict, so `s[0]` raised KeyError — caught
-                # by the per-account handler below, which meant every single
-                # scheduled tick failed for every account and nothing was ever
-                # synced automatically.
-                s = query_one(
-                    "SELECT history_days FROM user_settings WHERE user_id = ?", (uid,)
-                )
-                user_days = s.get("history_days") if s else None
-                if user_days == 0:
-                    user_days = 730  # 全部历史 → 最大范围
-                d = user_days if user_days is not None else SYNC_DAYS
-            else:
-                d = days
+            d = SYNC_DAYS if days is None else days
             # Never poke Garmin while it is rate-limiting us — that is exactly
             # what keeps the limit alive. Respect the persisted cooldown and sit
             # this tick out.
@@ -187,12 +173,7 @@ def _loop():
         try:
             if claim(interval=TICK_SECONDS):
                 try:
-                    # Explicitly the recent window, never the user's 历史范围:
-                    # that setting describes the manual 全量同步. Letting a
-                    # half-hourly tick re-pull "全部历史" meant 730 days x ~7
-                    # Garmin calls every 30 minutes, which is precisely what
-                    # kept the account in a 429 loop.
-                    sync_all_accounts(days=SYNC_DAYS, respect_schedule=True)
+                    sync_all_accounts(respect_schedule=True)
                 finally:
                     release()
         except Exception as e:  # noqa: BLE001 - the loop must outlive any single failure