fix: audio cleanup srcObject, diagnostics rate limit, orphan cleanup race (BUG-107, BUG-121, BUG-132)

BUG-107: cleanupAllAudioElements now calls pause() and sets
srcObject = null before removing elements from DOM, ensuring streams
are fully released during reconnection cleanup.

BUG-121: Diagnostics endpoint now has 5 req/min rate limit as
documented, preventing enumeration of internal topology.

BUG-132: DeleteOrphanedAttachments uses DELETE ... RETURNING stored_as
(atomic) instead of separate SELECT then DELETE, eliminating the race
where a file could be deleted after its attachment was linked.

Server/db/attachment_queries.go

@@ -159,16 +159,21 @@ func (d *DB) GetAttachmentsByMessageIDs(msgIDs []int64) (map[int64][]AttachmentI
 	return result, nil
 }
 
-// DeleteOrphanedAttachments removes attachment records where message_id IS NULL
-// and uploaded_at is older than the given cutoff time string (ISO 8601).
-// Returns the stored_as filenames of deleted records so the caller can remove files.
+// DeleteOrphanedAttachments atomically removes attachment records where
+// message_id IS NULL and uploaded_at is older than the given cutoff time
+// string (ISO 8601). Returns the stored_as filenames of deleted records
+// so the caller can remove files.
+//
+// BUG-132: Uses DELETE ... RETURNING to make select+delete atomic,
+// preventing a race where an attachment linked between SELECT and DELETE
+// would have its file deleted while the DB row survives.
 func (d *DB) DeleteOrphanedAttachments(cutoff string) ([]string, error) {
 	rows, err := d.sqlDB.Query(
-		`SELECT stored_as FROM attachments WHERE message_id IS NULL AND uploaded_at < ?`,
+		`DELETE FROM attachments WHERE message_id IS NULL AND uploaded_at < ? RETURNING stored_as`,
 		cutoff,
 	)
 	if err != nil {
-		return nil, fmt.Errorf("DeleteOrphanedAttachments query: %w", err)
+		return nil, fmt.Errorf("DeleteOrphanedAttachments: %w", err)
 	}
 	defer rows.Close() //nolint:errcheck
 
@@ -184,15 +189,5 @@ func (d *DB) DeleteOrphanedAttachments(cutoff string) ([]string, error) {
 		return nil, fmt.Errorf("DeleteOrphanedAttachments rows: %w", rows.Err())
 	}
 
-	if len(files) > 0 {
-		_, err = d.sqlDB.Exec(
-			`DELETE FROM attachments WHERE message_id IS NULL AND uploaded_at < ?`,
-			cutoff,
-		)
-		if err != nil {
-			return nil, fmt.Errorf("DeleteOrphanedAttachments delete: %w", err)
-		}
-	}
-
 	return files, nil
 }