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_open_cgo.go

@@ -0,0 +1,18 @@
+// 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/.
+
+//go:build cgo
+
+package sqlite
+
+import (
+	_ "github.com/mattn/go-sqlite3" // register sqlite3 database driver
+)
+
+const (
+	dbDriver      = "sqlite3"
+	commonOptions = "_fk=true&_rt=true&_cache_size=-65536&_sync=1&_txlock=immediate"
+)