chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

internal/db/sqlite/db_mtimes.go

@@ -0,0 +1,54 @@
+// Copyright (C) 2025 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package sqlite
+
+import (
+	"time"
+)
+
+func (s *DB) GetMtime(folder, name string) (ondisk, virtual time.Time) {
+	var res struct {
+		Ondisk  int64
+		Virtual int64
+	}
+	if err := s.stmt(`
+		SELECT m.ondisk, m.virtual FROM mtimes m
+		INNER JOIN folders o ON o.idx = m.folder_idx
+		WHERE o.folder_id = ? AND m.name = ?
+	`).Get(&res, folder, name); err != nil {
+		return time.Time{}, time.Time{}
+	}
+	return time.Unix(0, res.Ondisk), time.Unix(0, res.Virtual)
+}
+
+func (s *DB) PutMtime(folder, name string, ondisk, virtual time.Time) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	folderIdx, err := s.folderIdxLocked(folder)
+	if err != nil {
+		return wrap(err)
+	}
+	_, err = s.stmt(`
+		INSERT OR REPLACE INTO mtimes (folder_idx, name, ondisk, virtual)
+		VALUES (?, ?, ?, ?)
+	`).Exec(folderIdx, name, ondisk.UnixNano(), virtual.UnixNano())
+	return wrap(err)
+}
+
+func (s *DB) DeleteMtime(folder, name string) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	folderIdx, err := s.folderIdxLocked(folder)
+	if err != nil {
+		return wrap(err)
+	}
+	_, err = s.stmt(`
+		DELETE FROM mtimes
+		WHERE folder_idx = ? AND name = ?
+	`).Exec(folderIdx, name)
+	return wrap(err)
+}