Add external service support for sensitive media detection

- Add sensitiveMediaDetectionProxyUrl field to Meta model
- Create migration for new database field
- Update admin API endpoints to expose and update the proxy URL
- Modify AiService to support external API calls when proxy URL is configured
- Update frontend admin UI to add proxy URL configuration field
- Add i18n strings for proxy URL in English and Japanese

Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

packages/backend/migration/1731916961000-SensitiveMediaDetectionProxy.js

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export class SensitiveMediaDetectionProxy1731916961000 {
+    name = 'SensitiveMediaDetectionProxy1731916961000'
+
+    async up(queryRunner) {
+        await queryRunner.query(`ALTER TABLE "meta" ADD "sensitiveMediaDetectionProxyUrl" character varying(1024)`);
+    }
+
+    async down(queryRunner) {
+        await queryRunner.query(`ALTER TABLE "meta" DROP COLUMN "sensitiveMediaDetectionProxyUrl"`);
+    }
+}