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/billing/definition.py

@@ -29,13 +29,14 @@ billing_module = ModuleDefinition(
     code="billing",
     name="Billing & Subscriptions",
     description=(
-        "Subscription tier management, vendor billing, payment processing, "
-        "and invoice history. Integrates with Stripe for payment collection."
+        "Platform subscription management, vendor billing, and invoice history. "
+        "Uses the payments module for actual payment processing."
     ),
+    version="1.0.0",
+    requires=["payments"],  # Depends on payments module for payment processing
     features=[
         "subscription_management",  # Manage subscription tiers
         "billing_history",  # View invoices and payment history
-        "stripe_integration",  # Stripe payment processing
         "invoice_generation",  # Generate and download invoices
         "subscription_analytics",  # Subscription stats and metrics
         "trial_management",  # Manage vendor trial periods