feat(billing): add /billing/checkout-session via BillingService

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/core/config.py

@@ -94,11 +94,12 @@ class Settings(BaseSettings):
     STRIPE_SECRET_KEY: Optional[str] = None
     STRIPE_PUBLISHABLE_KEY: Optional[str] = None
     STRIPE_WEBHOOK_SECRET: Optional[str] = None
+    SELF_SERVE_ENABLED: bool = False
 
     @property
     def stripe_enabled(self) -> bool:
         """Check if Stripe is configured."""
-        return self.STRIPE_SECRET_KEY is not None and self.STRIPE_WEBHOOK_SECRET is not None
+        return bool(self.STRIPE_SECRET_KEY)
 
     # AI Flow Builder
     ANTHROPIC_API_KEY: Optional[str] = None