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-blocks.sql

@@ -0,0 +1,34 @@
+-- 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/.
+
+-- Block lists
+--
+-- The block lists are extracted from FileInfos and stored separately. This
+-- reduces the database size by reusing the same block list entry for all
+-- devices announcing the same file. Doing it for all block lists instead of
+-- using a size cutoff simplifies queries. Block lists are garbage collected
+-- "manually", not using a trigger as that was too performance impacting.
+CREATE TABLE IF NOT EXISTS blocklists (
+    blocklist_hash BLOB NOT NULL PRIMARY KEY,
+    blprotobuf BLOB NOT NULL
+) STRICT
+;
+
+-- Blocks
+--
+-- For all local files we store the blocks individually for quick lookup. A
+-- given block can exist in multiple blocklists and at multiple offsets in a
+-- blocklist.
+CREATE TABLE IF NOT EXISTS blocks (
+    hash BLOB NOT NULL,
+    blocklist_hash BLOB NOT NULL,
+    idx INTEGER NOT NULL,
+    offset INTEGER NOT NULL,
+    size INTEGER NOT NULL,
+    PRIMARY KEY (hash, blocklist_hash, idx),
+    FOREIGN KEY(blocklist_hash) REFERENCES blocklists(blocklist_hash) ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED
+) STRICT
+;