chore(db): use one SQLite database per folder (#10042)

This changes the database structure to use one database per folder, with
a small main database to coordinate. Reverts the prior change to buffer
all files in memory when pulling, meaning there is now a phase where the
WAL file will grow significantly, at least for initial sync of folders
with many directories.

---------

Co-authored-by: bt90 <btom1990@googlemail.com>

internal/db/sqlite/sql/schema/folder/50-mtimes.sql

@@ -0,0 +1,14 @@
+-- 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 (
+    name TEXT NOT NULL,
+    ondisk INTEGER NOT NULL, -- unix nanos
+    virtual INTEGER NOT NULL, -- unix nanos
+    PRIMARY KEY(name)
+) STRICT, WITHOUT ROWID
+;