Fix: add sync_lookback_days column to GarminConnectConfig model

This was missing from the earlier commit — tasks.py referenced
cfg.sync_lookback_days but the column wasn't defined in the SQLAlchemy model,
causing AttributeError when connecting or syncing Garmin.

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

backend/app/models/user.py

@@ -52,6 +52,7 @@ class GarminConnectConfig(Base):
     sync_enabled = Column(Boolean, default=True)
     sync_activities = Column(Boolean, default=True)
     sync_wellness = Column(Boolean, default=True)
+    sync_lookback_days = Column(Integer, default=30)   # -1 = all-time
     last_sync_at = Column(DateTime(timezone=True), nullable=True)
     last_sync_status = Column(String(512), nullable=True)
     created_at = Column(DateTime(timezone=True), default=now_utc)