feat: add platform type filter to subscription management page

Add a platform filter dropdown to the admin subscriptions view, allowing
filtering subscriptions by platform (Anthropic, OpenAI, Gemini, etc.)
through the group association.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/admin/subscriptions.ts

@@ -27,6 +27,7 @@ export async function list(
     status?: 'active' | 'expired' | 'revoked'
     user_id?: number
     group_id?: number
+    platform?: string
     sort_by?: string
     sort_order?: 'asc' | 'desc'
   },