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.go

@@ -0,0 +1,77 @@
+// 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 (
+	"sync"
+	"time"
+
+	"github.com/jmoiron/sqlx"
+	"github.com/syncthing/syncthing/internal/db"
+	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/thejerf/suture/v4"
+)
+
+type DB struct {
+	sql            *sqlx.DB
+	localDeviceIdx int64
+	updateLock     sync.Mutex
+
+	statementsMut sync.RWMutex
+	statements    map[string]*sqlx.Stmt
+	tplInput      map[string]any
+}
+
+var _ db.DB = (*DB)(nil)
+
+func (s *DB) Close() error {
+	s.updateLock.Lock()
+	s.statementsMut.Lock()
+	defer s.updateLock.Unlock()
+	defer s.statementsMut.Unlock()
+	for _, stmt := range s.statements {
+		stmt.Close()
+	}
+	return wrap(s.sql.Close())
+}
+
+func (s *DB) Service(maintenanceInterval time.Duration) suture.Service {
+	return newService(s, maintenanceInterval)
+}
+
+func (s *DB) ListFolders() ([]string, error) {
+	var res []string
+	err := s.stmt(`
+		SELECT folder_id FROM folders
+		ORDER BY folder_id
+	`).Select(&res)
+	return res, wrap(err)
+}
+
+func (s *DB) ListDevicesForFolder(folder string) ([]protocol.DeviceID, error) {
+	var res []string
+	err := s.stmt(`
+		SELECT d.device_id FROM counts s
+		INNER JOIN folders o ON o.idx = s.folder_idx
+		INNER JOIN devices d ON d.idx = s.device_idx
+		WHERE o.folder_id = ? AND s.count > 0 AND s.device_idx != {{.LocalDeviceIdx}}
+		GROUP BY d.device_id
+		ORDER BY d.device_id
+	`).Select(&res, folder)
+	if err != nil {
+		return nil, wrap(err)
+	}
+
+	devs := make([]protocol.DeviceID, len(res))
+	for i, s := range res {
+		devs[i], err = protocol.DeviceIDFromString(s)
+		if err != nil {
+			return nil, wrap(err)
+		}
+	}
+	return devs, nil
+}