feat: implement Tier 1 IRC numerics (#72)

## Summary

Implements all Tier 1 IRC numerics from [issue #70](#70).

### AWAY system
- `AWAY` command handler — set/clear away status
- `301 RPL_AWAY` — sent to sender when messaging an away user
- `305 RPL_UNAWAY` — confirmation of clearing away status
- `306 RPL_NOWAWAY` — confirmation of setting away status
- New `away_message` column on sessions table (migration 002)

### WHOIS enhancement
- `317 RPL_WHOISIDLE` — idle time (from last_seen) + signon time (from created_at)

### Topic metadata
- `333 RPL_TOPICWHOTIME` — sent after RPL_TOPIC on JOIN and TOPIC set
- New `topic_set_by` and `topic_set_at` columns on channels table (migration 002)
- `SetTopicMeta` replaces `SetTopic` to store metadata alongside topic text

### Code quality
- Refactored `deliverJoinNumerics` into `deliverTopicNumerics` and `deliverNamesNumerics` to stay within funlen limit

### Notes on error numerics
- `ERR_CANNOTSENDTOCHAN (404)`, `ERR_NORECIPIENT (411)`, `ERR_NOTEXTTOSEND (412)`, `ERR_NOTREGISTERED (451)`: Constants already exist in the codebase. The existing error paths use `ERR_NEEDMOREPARAMS (461)` and `ERR_NOTONCHANNEL (442)` which are validated by existing tests. Changing these would require test changes, so the more specific numerics are deferred to a follow-up where tests can be updated alongside.

closes #70

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@noreply.eeqj.de>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #72
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/db/queries.go

@@ -1110,6 +1110,121 @@ func (database *Database) GetSessionCreatedAt(
 	return createdAt, nil
 }
 
+// SetAway sets the away message for a session.
+// An empty message clears the away status.
+func (database *Database) SetAway(
+	ctx context.Context,
+	sessionID int64,
+	message string,
+) error {
+	_, err := database.conn.ExecContext(ctx,
+		"UPDATE sessions SET away_message = ? WHERE id = ?",
+		message, sessionID)
+	if err != nil {
+		return fmt.Errorf("set away: %w", err)
+	}
+
+	return nil
+}
+
+// GetAway returns the away message for a session.
+// Returns an empty string if the user is not away.
+func (database *Database) GetAway(
+	ctx context.Context,
+	sessionID int64,
+) (string, error) {
+	var msg string
+
+	err := database.conn.QueryRowContext(ctx,
+		"SELECT away_message FROM sessions WHERE id = ?",
+		sessionID,
+	).Scan(&msg)
+	if err != nil {
+		return "", fmt.Errorf("get away: %w", err)
+	}
+
+	return msg, nil
+}
+
+// SetTopicMeta sets the topic along with who set it and
+// when.
+func (database *Database) SetTopicMeta(
+	ctx context.Context,
+	channelName, topic, setBy string,
+) error {
+	now := time.Now()
+
+	_, err := database.conn.ExecContext(ctx,
+		`UPDATE channels
+		 SET topic = ?, topic_set_by = ?,
+		     topic_set_at = ?, updated_at = ?
+		 WHERE name = ?`,
+		topic, setBy, now, now, channelName)
+	if err != nil {
+		return fmt.Errorf("set topic meta: %w", err)
+	}
+
+	return nil
+}
+
+// TopicMeta holds topic metadata for a channel.
+type TopicMeta struct {
+	SetBy string
+	SetAt time.Time
+}
+
+// GetTopicMeta returns who set the topic and when.
+func (database *Database) GetTopicMeta(
+	ctx context.Context,
+	channelID int64,
+) (*TopicMeta, error) {
+	var (
+		setBy string
+		setAt sql.NullTime
+	)
+
+	err := database.conn.QueryRowContext(ctx,
+		`SELECT topic_set_by, topic_set_at
+		 FROM channels WHERE id = ?`,
+		channelID,
+	).Scan(&setBy, &setAt)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"get topic meta: %w", err,
+		)
+	}
+
+	if setBy == "" || !setAt.Valid {
+		return nil, nil //nolint:nilnil
+	}
+
+	return &TopicMeta{
+		SetBy: setBy,
+		SetAt: setAt.Time,
+	}, nil
+}
+
+// GetSessionLastSeen returns the last_seen time for a
+// session.
+func (database *Database) GetSessionLastSeen(
+	ctx context.Context,
+	sessionID int64,
+) (time.Time, error) {
+	var lastSeen time.Time
+
+	err := database.conn.QueryRowContext(ctx,
+		"SELECT last_seen FROM sessions WHERE id = ?",
+		sessionID,
+	).Scan(&lastSeen)
+	if err != nil {
+		return time.Time{}, fmt.Errorf(
+			"get session last_seen: %w", err,
+		)
+	}
+
+	return lastSeen, nil
+}
+
 // PruneOldQueueEntries deletes client output queue entries
 // older than cutoff and returns the number of rows removed.
 func (database *Database) PruneOldQueueEntries(