Add configurable sync_lookback_days for Garmin Connect

Users can now set how many days back the first sync fetches. -1 syncs all
history back to 2010; any positive value sets a rolling window. Values
over 365 show a rate-limit warning in the UI. The default remains 30 days.

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

backend/app/api/garmin_sync.py

@@ -18,6 +18,7 @@ class GarminConfigIn(BaseModel):
     sync_enabled: bool = True
     sync_activities: bool = True
     sync_wellness: bool = True
+    sync_lookback_days: int = 30         # days to look back on first sync; -1 = all-time
 
 
 class GarminConfigOut(BaseModel):
@@ -25,9 +26,10 @@ class GarminConfigOut(BaseModel):
     sync_enabled: bool
     sync_activities: bool
     sync_wellness: bool
+    sync_lookback_days: int
     last_sync_at: Optional[datetime]
     last_sync_status: Optional[str]
-    connected: bool                       # True when credentials exist
+    connected: bool
 
     class Config:
         from_attributes = True
@@ -45,14 +47,15 @@ async def get_config(
     if not cfg:
         return GarminConfigOut(
             email="", sync_enabled=False, sync_activities=True,
-            sync_wellness=True, last_sync_at=None, last_sync_status=None,
-            connected=False,
+            sync_wellness=True, sync_lookback_days=30,
+            last_sync_at=None, last_sync_status=None, connected=False,
         )
     return GarminConfigOut(
         email=cfg.email,
         sync_enabled=cfg.sync_enabled,
         sync_activities=cfg.sync_activities,
         sync_wellness=cfg.sync_wellness,
+        sync_lookback_days=cfg.sync_lookback_days if cfg.sync_lookback_days is not None else 30,
         last_sync_at=cfg.last_sync_at,
         last_sync_status=cfg.last_sync_status,
         connected=True,
@@ -92,6 +95,7 @@ async def save_config(
         cfg.sync_enabled = body.sync_enabled
         cfg.sync_activities = body.sync_activities
         cfg.sync_wellness = body.sync_wellness
+        cfg.sync_lookback_days = body.sync_lookback_days
         cfg.last_sync_status = "Credentials updated"
     else:
         cfg = GarminConnectConfig(
@@ -102,6 +106,7 @@ async def save_config(
             sync_enabled=body.sync_enabled,
             sync_activities=body.sync_activities,
             sync_wellness=body.sync_wellness,
+            sync_lookback_days=body.sync_lookback_days,
             last_sync_status="Connected",
         )
         db.add(cfg)
@@ -114,6 +119,7 @@ async def save_config(
         sync_enabled=cfg.sync_enabled,
         sync_activities=cfg.sync_activities,
         sync_wellness=cfg.sync_wellness,
+        sync_lookback_days=cfg.sync_lookback_days if cfg.sync_lookback_days is not None else 30,
         last_sync_at=cfg.last_sync_at,
         last_sync_status=cfg.last_sync_status,
         connected=True,