fix index in message model

db/models/message.py

@@ -34,7 +34,8 @@ class Message(SQLModel, table=True):
     # Use aware timestamps and TIMESTAMP WITH TIME ZONE
     created_at: datetime = Field(
         default_factory=aware_utcnow,
-        sa_column=Column(TIMESTAMP(timezone=True), nullable=False)
+        sa_column=Column(TIMESTAMP(timezone=True), nullable=False),
+        index=True
     )
 
     chat: Optional["Chat"] = Relationship(back_populates="messages")