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/30-indexids.sql

@@ -0,0 +1,24 @@
+-- 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/.
+
+-- indexids holds the index ID and maximum sequence for a given device and folder
+CREATE TABLE IF NOT EXISTS indexids (
+    device_idx INTEGER NOT NULL,
+    folder_idx INTEGER NOT NULL,
+    index_id TEXT NOT NULL COLLATE BINARY,
+    sequence INTEGER NOT NULL DEFAULT 0,
+    PRIMARY KEY(device_idx, folder_idx),
+    FOREIGN KEY(folder_idx) REFERENCES folders(idx) ON DELETE CASCADE,
+    FOREIGN KEY(device_idx) REFERENCES devices(idx) ON DELETE CASCADE
+) STRICT, WITHOUT ROWID
+;
+CREATE TRIGGER IF NOT EXISTS indexids_seq AFTER INSERT ON files
+BEGIN
+    INSERT INTO indexids (folder_idx, device_idx, index_id, sequence)
+        VALUES (NEW.folder_idx, NEW.device_idx, "", COALESCE(NEW.remote_sequence, NEW.sequence))
+        ON CONFLICT DO UPDATE SET sequence = COALESCE(NEW.remote_sequence, NEW.sequence);
+END
+;