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/tests/test_scheduler.py

@@ -187,12 +187,11 @@ class TestEndpoints:
 
 
 class TestSyncWindowResolution:
-    """Which window each account is synced over.
+    """Auto-sync must be blind to the manual sync range.
 
-    `query_one` returns a dict, so reading the saved 历史范围 as `s[0]` raised
-    KeyError — swallowed by the per-account handler, which meant every
-    scheduled tick failed for every account that had ever opened 设置, and
-    nothing was synced automatically at all.
+    They used to share `history_days`: one setting on the 设置 page that only
+    took effect on the 数据同步 page, and that a half-hourly tick also used to
+    re-pull 730 days with. Separating them is the point.
     """
 
     def _account(self, user, monkeypatch):
@@ -208,34 +207,23 @@ class TestSyncWindowResolution:
         monkeypatch.setattr(garmin_svc, "sync_data", record)
         return seen
 
-    def test_saved_history_days_is_honoured(self, db, user, monkeypatch):
-        from services import settings as settings_svc
-
+    def test_the_default_window_is_used(self, db, user, monkeypatch):
         seen = self._account(user, monkeypatch)
-        settings_svc.save_settings(user["id"], {"historyDays": 90})
-
         results = scheduler.sync_all_accounts()
         assert [r["status"] for r in results] == ["success"]
-        assert seen["days"] == 90
+        assert seen["days"] == scheduler.SYNC_DAYS
 
-    def test_full_history_becomes_the_maximum(self, db, user, monkeypatch):
+    def test_the_manual_range_is_ignored(self, db, user, monkeypatch):
+        """全部历史 on the sync page must not turn every tick into 730 days."""
         from services import settings as settings_svc
 
         seen = self._account(user, monkeypatch)
         settings_svc.save_settings(user["id"], {"historyDays": 0})
 
-        scheduler.sync_all_accounts()
-        assert seen["days"] == 730
-
-    def test_the_scheduled_tick_ignores_the_history_setting(
-        self, db, user, monkeypatch
-    ):
-        """历史范围 is the window for the manual 全量同步. A half-hourly tick
-        re-pulling 全部历史 is what kept the account in a 429 loop."""
-        from services import settings as settings_svc
-
-        seen = self._account(user, monkeypatch)
-        settings_svc.save_settings(user["id"], {"historyDays": 0, "autoSync": True})
-
-        scheduler.sync_all_accounts(days=scheduler.SYNC_DAYS, respect_schedule=True)
+        scheduler.sync_all_accounts(respect_schedule=True)
         assert seen["days"] == scheduler.SYNC_DAYS
+
+    def test_an_explicit_override_still_wins(self, db, user, monkeypatch):
+        seen = self._account(user, monkeypatch)
+        scheduler.sync_all_accounts(days=30)
+        assert seen["days"] == 30