license: change from MIT to AGPLv3

Update LICENSE file and add AGPL header to all source files.

AGPLv3 ensures that if someone runs Kafka as a network service and
modifies it, they must release their source code under the same license.

internal/middleware/ratelimit_global.go

@@ -1,3 +1,19 @@
+// kafka — a privacy-respecting metasearch engine
+// Copyright (C) 2026-present metamorphosis-dev
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
 package middleware
 
 import (