backend: implement agents and connect them with chats

backend/database.py

@@ -57,8 +57,7 @@ class Chat(Base):
     ordering: Mapped[Optional[int]]
 
     # For indexing purposes, some information is taken from the last chat message.
-    # Preview is the (possibly cropped) content,
-    # time is the timestamp of the last event.
+    # Preview is the (possibly cropped) content, time is the timestamp of the message.
     last_message_preview: Mapped[Optional[str]]
     last_message_time: Mapped[Optional[int]]
 
@@ -81,7 +80,7 @@ class Message(Base):
 
     # Timestamp of this message
     time: Mapped[int]
-    # Identifier of the sender: user / system / agent-$ID
+    # Identifier of the sender: user / system / $AGENT_ID
     sender: Mapped[str]
 
     # Predefined set of message types, interpreted as 'text' when empty.