Implement robust reconnection system, updates, optimize typing

backend/models.py

@@ -283,5 +283,20 @@ class DMReactionResponse(BaseModel):
         from_attributes = True
 
 
+class UpdateLog(Base):
+    """Stores update sequence numbers and updates for gap detection"""
+    __tablename__ = "update_log"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("user.id"), nullable=False, index=True)
+    sequence = Column(Integer, nullable=False, index=True)
+    updates = Column(Text, nullable=False)  # JSON array of updates
+    timestamp = Column(DateTime, default=datetime.now, index=True)
+
+    __table_args__ = (
+        UniqueConstraint("user_id", "sequence", name="uq_user_sequence"),
+    )
+
+
 # Tables are now created through Alembic migrations
 # Base.metadata.create_all(bind=engine)