Bind history to the recipient session and channel, not the name (closes #148)

History was selected by msg_to, so whoever next held a released nick, or
created a deleted channel again under its old name, could read the
earlier holder's messages. Each message row now records the session a
DM or INVITE was sent to (to_session_id) and the channel a channel
message was sent to (to_channel_id), and GET /api/v1/history selects by
the one that applies, using its index. A session's DM history now also
follows it across nick changes. A DM or channel message whose recipient
is deleted before it is stored gets the unknown nick or channel error.

Model: opus-5-5

internal/ircserver/relay_test.go

@@ -28,7 +28,7 @@ func (env *testEnv) queueMessage(
 	}
 
 	msgID, _, err := env.database.InsertMessage(
-		ctx, command, from, to, nil, bodyJSON, nil,
+		ctx, command, from, to, 0, 0, nil, bodyJSON, nil,
 	)
 	if err != nil {
 		t.Fatalf("insert message: %v", err)