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_kv.go

@@ -0,0 +1,78 @@
+// 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 (
+	"iter"
+
+	"github.com/jmoiron/sqlx"
+	"github.com/syncthing/syncthing/internal/db"
+)
+
+func (s *DB) GetKV(key string) ([]byte, error) {
+	var val []byte
+	if err := s.stmt(`
+		SELECT value FROM kv
+		WHERE key = ?
+	`).Get(&val, key); err != nil {
+		return nil, wrap(err)
+	}
+	return val, nil
+}
+
+func (s *DB) PutKV(key string, val []byte) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	_, err := s.stmt(`
+		INSERT OR REPLACE INTO kv (key, value)
+		VALUES (?, ?)
+	`).Exec(key, val)
+	return wrap(err)
+}
+
+func (s *DB) DeleteKV(key string) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	_, err := s.stmt(`
+		DELETE FROM kv WHERE key = ?
+	`).Exec(key)
+	return wrap(err)
+}
+
+func (s *DB) PrefixKV(prefix string) (iter.Seq[db.KeyValue], func() error) {
+	var rows *sqlx.Rows
+	var err error
+	if prefix == "" {
+		rows, err = s.stmt(`SELECT key, value FROM kv`).Queryx()
+	} else {
+		end := prefixEnd(prefix)
+		rows, err = s.stmt(`
+			SELECT key, value FROM kv
+			WHERE key >= ? AND key < ?
+		`).Queryx(prefix, end)
+	}
+	if err != nil {
+		return func(_ func(db.KeyValue) bool) {}, func() error { return err }
+	}
+
+	return func(yield func(db.KeyValue) bool) {
+			defer rows.Close()
+			for rows.Next() {
+				var key string
+				var val []byte
+				if err = rows.Scan(&key, &val); err != nil {
+					return
+				}
+				if !yield(db.KeyValue{Key: key, Value: val}) {
+					return
+				}
+			}
+			err = rows.Err()
+		}, func() error {
+			return err
+		}
+}