Add landing page, referral system, usage quotas, search API management, and yearly pricing

- Separate workspace landing from login for better UX
- Referral system rewards both parties with Pro days
- Quota enforcement prevents abuse without breaking endpoints
- 7-day free trial with auto-downgrade on expiry
- Admin-managed search provider config (SearXNG, Bing)
- 15% discount on annual subscriptions
- MCP search server wrapping opencode search
- Fix discovery module field name mismatch causing 422

backend/app/models/__init__.py

@@ -14,6 +14,8 @@ from .system_config import SystemConfig
 from .translation_quota import TranslationQuota
 from .certification import Certification, CertType, CertStatus
 from .invoice import Invoice, InvoiceType, InvoiceStatus
+from .referral import ReferralCode, Referral
+from .search_provider import SearchProvider
 
 __all__ = [
     "User", "Product",
@@ -32,4 +34,6 @@ __all__ = [
     "TranslationQuota",
     "Certification", "CertType", "CertStatus",
     "Invoice", "InvoiceType", "InvoiceStatus",
+    "ReferralCode", "Referral",
+    "SearchProvider",
 ]