refactor: 30-day defaults, time-based message expiry

- Change QUEUE_MAX_AGE default from 48h to 30 days (2592000s)
- Replace count-based MAX_HISTORY with time-based MESSAGE_MAX_AGE
  (default 30 days) — messages older than the configured age are
  pruned instead of keeping a fixed count per target
- Update README to reflect new defaults and time-based expiry

internal/db/queries.go

@@ -1118,84 +1118,23 @@ func (database *Database) PruneOldQueueEntries(
 	return deleted, nil
 }
 
-// RotateChannelMessages enforces MAX_HISTORY per channel
-// by deleting the oldest messages beyond the limit for
-// each msg_to target. Returns the total number of rows
-// removed.
-func (database *Database) RotateChannelMessages(
+// PruneOldMessages deletes messages older than cutoff and
+// returns the number of rows removed.
+func (database *Database) PruneOldMessages(
 	ctx context.Context,
-	maxHistory int,
+	cutoff time.Time,
 ) (int64, error) {
-	if maxHistory <= 0 {
-		return 0, nil
-	}
-
-	// Find distinct targets that have messages.
-	rows, err := database.conn.QueryContext(ctx,
-		`SELECT msg_to, COUNT(*) AS cnt
-		 FROM messages
-		 WHERE msg_to != ''
-		 GROUP BY msg_to
-		 HAVING cnt > ?`,
-		maxHistory,
+	res, err := database.conn.ExecContext(ctx,
+		"DELETE FROM messages WHERE created_at < ?",
+		cutoff,
 	)
 	if err != nil {
 		return 0, fmt.Errorf(
-			"list targets for rotation: %w", err,
+			"prune old messages: %w", err,
 		)
 	}
 
-	defer func() { _ = rows.Close() }()
+	deleted, _ := res.RowsAffected()
 
-	type targetCount struct {
-		target string
-		count  int64
-	}
-
-	var targets []targetCount
-
-	for rows.Next() {
-		var entry targetCount
-
-		err = rows.Scan(&entry.target, &entry.count)
-		if err != nil {
-			return 0, fmt.Errorf(
-				"scan target count: %w", err,
-			)
-		}
-
-		targets = append(targets, entry)
-	}
-
-	err = rows.Err()
-	if err != nil {
-		return 0, fmt.Errorf("rows error: %w", err)
-	}
-
-	var totalDeleted int64
-
-	for _, entry := range targets {
-		res, delErr := database.conn.ExecContext(ctx,
-			`DELETE FROM messages
-			 WHERE msg_to = ?
-			   AND id NOT IN (
-			     SELECT id FROM messages
-			     WHERE msg_to = ?
-			     ORDER BY id DESC
-			     LIMIT ?
-			   )`,
-			entry.target, entry.target, maxHistory,
-		)
-		if delErr != nil {
-			return totalDeleted, fmt.Errorf(
-				"rotate messages for %s: %w",
-				entry.target, delErr,
-			)
-		}
-
-		deleted, _ := res.RowsAffected()
-		totalDeleted += deleted
-	}
-
-	return totalDeleted, nil
+	return deleted, nil
 }