Add last_discovery_run to UserSettings model

Column was added via SQL migration but missing from the SQLAlchemy model
definition, causing AttributeError when the discovery status endpoint
accesses s.last_discovery_run.

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

backend/models.py

@@ -124,6 +124,7 @@ class UserSettings(Base):
     use_oauth2 = Column(Boolean, default=False)
     sync_interval_hours = Column(Integer, default=0)   # 0 = disabled, 6/12/24 = auto-sync interval
     subtitle_langs = Column(String, default="")        # "" = disabled, "en", "en,sv", etc.
+    last_discovery_run = Column(DateTime, nullable=True, default=None)
 
 
 class DiscoveryQueue(Base):