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/sql/schema/50-mtimes.sql

@@ -0,0 +1,16 @@
+-- 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/.
+
+--- Backing for the MtimeFS
+CREATE TABLE IF NOT EXISTS mtimes (
+    folder_idx INTEGER NOT NULL,
+    name TEXT NOT NULL,
+    ondisk INTEGER NOT NULL, -- unix nanos
+    virtual INTEGER NOT NULL, -- unix nanos
+    PRIMARY KEY(folder_idx, name),
+    FOREIGN KEY(folder_idx) REFERENCES folders(idx) ON DELETE CASCADE
+) STRICT, WITHOUT ROWID
+;