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
those ids. A session's DM history now also follows it across nick
changes.

Model: opus-5-5

internal/service/service_test.go

@@ -164,7 +164,7 @@ func TestFanOut(t *testing.T) {
 	body, _ := json.Marshal([]string{"hello"}) //nolint:errchkjson
 
 	dbID, uuid, err := env.svc.FanOut(
-		ctx, irc.CmdPrivmsg, "alice", "#test",
+		ctx, irc.CmdPrivmsg, "alice", "#test", 0, 0,
 		nil, body, nil,
 		[]int64{sid1, sid2},
 	)