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/db_schema.go

@@ -0,0 +1,88 @@
+// 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 (
+	"embed"
+	"io/fs"
+	"strings"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/build"
+)
+
+const currentSchemaVersion = 1
+
+//go:embed sql/**
+var embedded embed.FS
+
+func (s *DB) runScripts(glob string, filter ...func(s string) bool) error {
+	scripts, err := fs.Glob(embedded, glob)
+	if err != nil {
+		return wrap(err)
+	}
+
+	tx, err := s.sql.Begin()
+	if err != nil {
+		return wrap(err)
+	}
+	defer tx.Rollback() //nolint:errcheck
+
+nextScript:
+	for _, scr := range scripts {
+		for _, fn := range filter {
+			if !fn(scr) {
+				l.Debugln("Skipping script", scr)
+				continue nextScript
+			}
+		}
+		l.Debugln("Executing script", scr)
+		bs, err := fs.ReadFile(embedded, scr)
+		if err != nil {
+			return wrap(err, scr)
+		}
+		// SQLite requires one statement per exec, so we split the init
+		// files on lines containing only a semicolon and execute them
+		// separately. We require it on a separate line because there are
+		// also statement-internal semicolons in the triggers.
+		for _, stmt := range strings.Split(string(bs), "\n;") {
+			if _, err := tx.Exec(stmt); err != nil {
+				return wrap(err, stmt)
+			}
+		}
+	}
+
+	return wrap(tx.Commit())
+}
+
+type schemaVersion struct {
+	SchemaVersion    int
+	AppliedAt        int64
+	SyncthingVersion string
+}
+
+func (s *schemaVersion) AppliedTime() time.Time {
+	return time.Unix(0, s.AppliedAt)
+}
+
+func (s *DB) setAppliedSchemaVersion(ver int) error {
+	_, err := s.stmt(`
+		INSERT OR IGNORE INTO schemamigrations (schema_version, applied_at, syncthing_version)
+		VALUES (?, ?, ?)
+	`).Exec(ver, time.Now().UnixNano(), build.LongVersion)
+	return wrap(err)
+}
+
+func (s *DB) getAppliedSchemaVersion() (schemaVersion, error) {
+	var v schemaVersion
+	err := s.stmt(`
+		SELECT schema_version as schemaversion, applied_at as appliedat, syncthing_version as syncthingversion FROM schemamigrations
+		ORDER BY schema_version DESC
+		LIMIT 1
+	`).Get(&v)
+	return v, wrap(err)
+}