feat: add GET /api/provision/images endpoint

- Add handleGetOsImages handler in provision.ts
- Add getOsImages method in ProvisioningService
- Add route in index.ts
- Returns key, name, family, is_default for each OS image

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/provisioning-service.ts

@@ -371,6 +371,13 @@ export class ProvisioningService {
     return { balance_krw: balance, user_id: user.id };
   }
 
+  /**
+   * Get available OS images
+   */
+  async getOsImages() {
+    return this.repo.getActiveOsImages();
+  }
+
   /**
    * Delete a server (terminate) - requires telegram_id for authorization
    */