Fix sync window: respect lookback_days even after first sync

Previously, once last_sync_at was set the incremental path always used
since-1d as start_date, ignoring lookback_days entirely. Increasing the
lookback setting had no effect on already-synced instances.

Fix: take min(since-1d, today-lookback_days) so the window always covers
at least the configured lookback period, whether or not a prior sync ran.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/garmin_connect_sync.py

@@ -91,8 +91,11 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
         # All-time: full pull on first sync, incremental thereafter
         start_date = (since - timedelta(days=1)).date() if since else date(2010, 1, 1)
     elif since:
-        # Incremental: one day overlap to catch any late-arriving activities
-        start_date = (since - timedelta(days=1)).date()
+        # Use whichever is earlier: one day before last sync OR the configured lookback
+        # window. This ensures increasing lookback_days actually fetches older data.
+        incremental = (since - timedelta(days=1)).date()
+        lookback    = date.today() - timedelta(days=max(lookback_days, 1))
+        start_date  = min(incremental, lookback)
     else:
         start_date = date.today() - timedelta(days=max(lookback_days, 1))
     end_date = date.today()
@@ -194,8 +197,11 @@ def sync_wellness(garmin, user_id: int, since: Optional[datetime], db,
     if lookback_days == -1:
         start_date = (since - timedelta(days=1)).date() if since else date(2010, 1, 1)
     elif since:
-        # Incremental: one day overlap to catch any late-arriving wellness data
-        start_date = (since - timedelta(days=1)).date()
+        # Use whichever is earlier: one day before last sync OR the configured lookback
+        # window. This ensures increasing lookback_days actually fetches older data.
+        incremental = (since - timedelta(days=1)).date()
+        lookback    = date.today() - timedelta(days=max(lookback_days, 1))
+        start_date  = min(incremental, lookback)
     else:
         start_date = date.today() - timedelta(days=max(lookback_days, 1))
     days = (date.today() - start_date).days + 1