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/folderdb_mtimes.go

@@ -0,0 +1,45 @@
+// 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/.
+
+package sqlite
+
+import (
+	"time"
+)
+
+func (s *folderDB) GetMtime(name string) (ondisk, virtual time.Time) {
+	var res struct {
+		Ondisk  int64
+		Virtual int64
+	}
+	if err := s.stmt(`
+		SELECT m.ondisk, m.virtual FROM mtimes m
+		WHERE m.name = ?
+	`).Get(&res, name); err != nil {
+		return time.Time{}, time.Time{}
+	}
+	return time.Unix(0, res.Ondisk), time.Unix(0, res.Virtual)
+}
+
+func (s *folderDB) PutMtime(name string, ondisk, virtual time.Time) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	_, err := s.stmt(`
+		INSERT OR REPLACE INTO mtimes (name, ondisk, virtual)
+		VALUES (?, ?, ?)
+	`).Exec(name, ondisk.UnixNano(), virtual.UnixNano())
+	return wrap(err)
+}
+
+func (s *folderDB) DeleteMtime(name string) error {
+	s.updateLock.Lock()
+	defer s.updateLock.Unlock()
+	_, err := s.stmt(`
+		DELETE FROM mtimes
+		WHERE name = ?
+	`).Exec(name)
+	return wrap(err)
+}