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/dev_tools/definition.py

@@ -17,6 +17,7 @@ dev_tools_module = ModuleDefinition(
     code="dev-tools",
     name="Developer Tools",
     description="Component library and icon browser for development.",
+    version="1.0.0",
     features=[
         "component_library",  # UI component browser
         "icon_browser",  # Icon library browser
@@ -29,6 +30,7 @@ dev_tools_module = ModuleDefinition(
         FrontendType.VENDOR: [],  # No vendor menu items
     },
     is_core=False,
+    is_internal=True,  # Internal module - admin-only, not customer-facing
 )