feat: credit-based billing system

- New DB models: credit_packages, subscription_plans, user_credits, credit_consumptions, credit_purchases
- CreditService: balance, deduct, add_credits, grant_free_trial, history
- User API: /api/v1/credits/* (balance/history/packages/purchase/subscribe)
- Admin API: /api/v1/admin/credit-* (CRUD packages/plans, user credits, consumptions)
- PaymentService.create_credit_order + handle_callback for credit purchases
- Credit deduction on: discovery, translate, marketing, ai_chat, followup
- Free trial 30 credits on registration
- Documentation: docs/CREDIT_SYSTEM.md

backend/app/services/discovery.py

@@ -259,7 +259,7 @@ URL: {company_url}
             return json.loads(text)
         except json.JSONDecodeError:
             import re
-             brace = text.find("{")
+            brace = text.find("{")
             end = text.rfind("}")
             if brace >= 0 and end > brace:
                 try: