fix: remove dead code, fold migration 002 into 001, implement error numerics

- Remove unused GetAwayByNick() from queries.go
- Delete migration 002; fold away_message and topic_set_by/topic_set_at
  columns into migration 001 (pre-1.0 policy: single migration file)
- PRIVMSG/NOTICE missing target now sends 411 ERR_NORECIPIENT
- PRIVMSG/NOTICE missing body now sends 412 ERR_NOTEXTTOSEND
- Non-member channel send now sends 404 ERR_CANNOTSENDTOCHAN
- Auth failure now returns 451 ERR_NOTREGISTERED numeric in response
- Update test assertions to match correct IRC numerics

internal/db/queries.go

@@ -1146,25 +1146,6 @@ func (database *Database) GetAway(
 	return msg, nil
 }
 
-// GetAwayByNick returns the away message for a nick.
-// Returns an empty string if the user is not away.
-func (database *Database) GetAwayByNick(
-	ctx context.Context,
-	nick string,
-) (string, error) {
-	var msg string
-
-	err := database.conn.QueryRowContext(ctx,
-		"SELECT away_message FROM sessions WHERE nick = ?",
-		nick,
-	).Scan(&msg)
-	if err != nil {
-		return "", fmt.Errorf("get away by nick: %w", err)
-	}
-
-	return msg, nil
-}
-
 // SetTopicMeta sets the topic along with who set it and
 // when.
 func (database *Database) SetTopicMeta(