Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

models/db/driver_sqlite_mattn.go

@@ -0,0 +1,34 @@
+//go:build sqlite
+
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package db
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func init() {
+	setting.SupportedDatabaseTypes = append(setting.SupportedDatabaseTypes, "sqlite3")
+	makeSQLiteConnStr = makeSQLiteConnStrMattnCGO
+}
+
+func makeSQLiteConnStrMattnCGO(opts SQLiteConnStrOptions) (string, string, error) {
+	var params []string
+	params = append(params, "cache=shared")
+	params = append(params, "mode=rwc")
+	params = append(params, "_busy_timeout="+strconv.Itoa(opts.BusyTimeout))
+	params = append(params, "_txlock=immediate")
+	if opts.JournalMode != "" {
+		params = append(params, "_journal_mode="+opts.JournalMode)
+	}
+	connStr := fmt.Sprintf("file:%s?%s", opts.FilePath, strings.Join(params, "&"))
+	return "sqlite3", connStr, nil
+}