refactor: migrate vendor billing, invoices, payments to module auto-discovery

Billing module:
- Create vendor_checkout.py (checkout, portal, cancel, reactivate, change-tier)
- Create vendor_addons.py (addon management routes)
- Update vendor.py to aggregate new routers

Orders module:
- Create vendor_invoices.py (invoice settings, CRUD, PDF generation)
- Update vendor.py to aggregate invoices router

Payments module:
- Restructure routes from routes/ to routes/api/
- Add require_module_access dependency
- Set is_self_contained=True for auto-discovery

Remove legacy files:
- app/api/v1/vendor/billing.py
- app/api/v1/vendor/invoices.py
- app/api/v1/vendor/payments.py

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

app/modules/payments/routes/api/admin.py

@@ -0,0 +1,52 @@
+# app/modules/payments/routes/api/admin.py
+"""
+Admin routes for payments module.
+
+Provides routes for:
+- Payment gateway configuration
+- Transaction monitoring
+- Refund management
+"""
+
+import logging
+
+from fastapi import APIRouter, Depends
+
+from app.api.deps import require_module_access
+
+admin_router = APIRouter(
+    prefix="/payments",
+    dependencies=[Depends(require_module_access("payments"))],
+)
+logger = logging.getLogger(__name__)
+
+
+@admin_router.get("/gateways")
+async def list_gateways():
+    """List configured payment gateways."""
+    # TODO: Implement gateway listing
+    return {
+        "gateways": [
+            {"code": "stripe", "name": "Stripe", "enabled": True},
+            {"code": "paypal", "name": "PayPal", "enabled": False},
+            {"code": "bank_transfer", "name": "Bank Transfer", "enabled": True},
+        ]
+    }
+
+
+@admin_router.get("/transactions")
+async def list_transactions():
+    """List recent transactions across all gateways."""
+    # TODO: Implement transaction listing
+    return {"transactions": [], "total": 0}
+
+
+@admin_router.post("/refunds/{transaction_id}")
+async def issue_refund(transaction_id: str, amount: float | None = None):
+    """Issue a refund for a transaction."""
+    # TODO: Implement refund logic
+    return {
+        "status": "pending",
+        "transaction_id": transaction_id,
+        "refund_amount": amount,
+    }