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/70-kv.sql

@@ -0,0 +1,13 @@
+-- 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/.
+
+--- Simple KV store. This backs the "miscDB" we use for certain minor pieces
+--  of data.
+CREATE TABLE IF NOT EXISTS kv (
+    key TEXT NOT NULL PRIMARY KEY COLLATE BINARY,
+    value BLOB NOT NULL
+) STRICT
+;