feat: implement three-tier module classification and framework layer

Module Classification:
- Core (4): core, tenancy, cms, customers - always enabled
- Optional (7): payments, billing, inventory, orders, marketplace, analytics, messaging
- Internal (2): dev-tools, monitoring - admin-only

Key Changes:
- Rename platform-admin module to tenancy
- Promote CMS and Customers to core modules
- Create new payments module (gateway abstractions)
- Add billing→payments and orders→payments dependencies
- Mark dev-tools and monitoring as internal modules

New Infrastructure:
- app/modules/events.py: Module event bus (ENABLED, DISABLED, STARTUP, SHUTDOWN)
- app/modules/migrations.py: Module-specific migration discovery
- app/core/observability.py: Health checks, Prometheus metrics, Sentry integration

Enhanced ModuleDefinition:
- version, is_internal, permissions
- config_schema, default_config
- migrations_path
- Lifecycle hooks: on_enable, on_disable, on_startup, health_check

New Registry Functions:
- get_optional_module_codes(), get_internal_module_codes()
- is_core_module(), is_internal_module()
- get_modules_by_tier(), get_module_tier()

Migrations:
- zc*: Rename platform-admin to tenancy
- zd*: Ensure CMS and Customers enabled for all platforms

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

app/modules/payments/routes/vendor.py

@@ -0,0 +1,40 @@
+# app/modules/payments/routes/vendor.py
+"""
+Vendor routes for payments module.
+
+Provides routes for:
+- Payment method management
+- Transaction history
+"""
+
+from fastapi import APIRouter
+
+vendor_router = APIRouter(prefix="/payments", tags=["Payments (Vendor)"])
+
+
+@vendor_router.get("/methods")
+async def list_payment_methods():
+    """List saved payment methods for the vendor."""
+    # TODO: Implement payment method listing
+    return {"payment_methods": []}
+
+
+@vendor_router.post("/methods")
+async def add_payment_method():
+    """Add a new payment method."""
+    # TODO: Implement payment method creation
+    return {"status": "created", "id": "pm_xxx"}
+
+
+@vendor_router.delete("/methods/{method_id}")
+async def remove_payment_method(method_id: str):
+    """Remove a saved payment method."""
+    # TODO: Implement payment method deletion
+    return {"status": "deleted", "id": method_id}
+
+
+@vendor_router.get("/transactions")
+async def list_vendor_transactions():
+    """List transactions for the vendor."""
+    # TODO: Implement transaction listing
+    return {"transactions": [], "total": 0}