refactor: use modernc sqlite driver as default (#37562)

The mattn driver is still kept, can be enabled by
TAGS="sqlite_mattn sqlite_unlock_notify"

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>

models/db/driver_sqlite_modernc.go

@@ -0,0 +1,41 @@
+//go:build !sqlite_mattn
+
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// modernc driver is chosen as the default one (compared to mattn, ncruces)
+// * mattn was used as default, but it requires CGO
+// * the CI times are almost the same for these three (race detector must be disabled)
+// * modernc increases the binary size about 2MB, ncruces increases about 7MB
+// * compiling time: modernc is slightly slower than mattn, ncruces is the slowest
+
+package db
+
+import (
+	"database/sql"
+	"fmt"
+	"strings"
+
+	"modernc.org/sqlite"
+)
+
+func init() {
+	// this driver contains huge amount of Golang code, so it is much slower when "-race" check is enabled.
+	registerSQLiteConnStrMaker(makeSQLiteConnStrModerncCCGO)
+	sql.Register(sqlDriverSQLite3, &sqlite.Driver{})
+}
+
+func makeSQLiteConnStrModerncCCGO(opts SQLiteConnStrOptions) (string, string, error) {
+	var params []string
+	// TODO: there is a changed behavior from mattn driver:
+	// * mattn driver can wait for pretty long time for concurrent accesses (not limited by the busy timeout)
+	// * but other drivers will report something like "database is locked (5) (SQLITE_BUSY)" if the timeout is reached
+	// Maybe we need to relax the busy timeout to a reasonable long time in the future
+	params = append(params, fmt.Sprintf("_pragma=busy_timeout(%d)", opts.BusyTimeout))
+	params = append(params, "_txlock=immediate")
+	if opts.JournalMode != "" {
+		params = append(params, fmt.Sprintf("_pragma=journal_mode(%s)", opts.JournalMode))
+	}
+	connStr := fmt.Sprintf("file:%s?%s", opts.FilePath, strings.Join(params, "&"))
+	return sqlDriverSQLite3, connStr, nil
+}