#6: implement IMAP IDLE (RFC 2177) and CONDSTORE (RFC 7162)

services/protocols/internal/application/port/flag_repository.go

@@ -16,6 +16,8 @@ const (
 type FlagRepository interface {
 	// SetFlags applies op with flags to the message identified by
 	// (folderID, uid). Add/Del insert/delete individual message_flags rows;
-	// Set replaces the message's full flag set in one transaction.
-	SetFlags(ctx context.Context, folderID string, uid uint32, op FlagOp, flags []string) error
+	// Set replaces the message's full flag set in one transaction. If
+	// unchangedSince > 0 and the message's current modseq exceeds it, the update
+	// is skipped and updated returns false.
+	SetFlags(ctx context.Context, folderID string, uid uint32, op FlagOp, flags []string, unchangedSince uint64) (updated bool, err error)
 }