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/__init__.py

@@ -4,6 +4,16 @@ Modular Platform Architecture.
 
 This package provides a module system for enabling/disabling feature bundles per platform.
 
+Three-Tier Classification:
+1. CORE MODULES (4) - Always enabled, cannot be disabled
+   - core, tenancy, cms, customers
+
+2. OPTIONAL MODULES (7) - Can be enabled/disabled per platform
+   - payments, billing, inventory, orders, marketplace, analytics, messaging
+
+3. INTERNAL MODULES (2) - Admin-only tools, not customer-facing
+   - dev-tools, monitoring
+
 Module Hierarchy:
     Global (SaaS Provider)
     └── Platform (Business Product - OMS, Loyalty, etc.)
@@ -11,7 +21,8 @@ Module Hierarchy:
             ├── Routes (API + Page routes)
             ├── Services (Business logic)
             ├── Menu Items (Sidebar entries)
-            └── Templates (UI components)
+            ├── Templates (UI components)
+            └── Migrations (Module-specific)
 
 Modules vs Features:
 - Features: Granular capabilities (e.g., analytics_dashboard, letzshop_sync)
@@ -22,7 +33,8 @@ Modules vs Features:
 Usage:
     from app.modules import module_service
     from app.modules.base import ModuleDefinition
-    from app.modules.registry import MODULES
+    from app.modules.registry import MODULES, CORE_MODULES, OPTIONAL_MODULES
+    from app.modules.events import module_event_bus, ModuleEvent
 
     # Check if module is enabled for platform
     if module_service.is_module_enabled(platform_id, "billing"):
@@ -33,15 +45,55 @@ Usage:
 
     # Get all enabled modules for platform
     modules = module_service.get_platform_modules(platform_id)
+
+    # Subscribe to module events
+    @module_event_bus.subscribe(ModuleEvent.ENABLED)
+    def on_enabled(data):
+        print(f"Module {data.module_code} enabled")
 """
 
 from app.modules.base import ModuleDefinition
-from app.modules.registry import MODULES
+from app.modules.registry import (
+    MODULES,
+    CORE_MODULES,
+    OPTIONAL_MODULES,
+    INTERNAL_MODULES,
+    get_core_module_codes,
+    get_optional_module_codes,
+    get_internal_module_codes,
+    get_module_tier,
+    is_core_module,
+    is_internal_module,
+)
 from app.modules.service import ModuleService, module_service
+from app.modules.events import (
+    ModuleEvent,
+    ModuleEventData,
+    ModuleEventBus,
+    module_event_bus,
+)
 
 __all__ = [
+    # Core types
     "ModuleDefinition",
+    # Module dictionaries
     "MODULES",
+    "CORE_MODULES",
+    "OPTIONAL_MODULES",
+    "INTERNAL_MODULES",
+    # Helper functions
+    "get_core_module_codes",
+    "get_optional_module_codes",
+    "get_internal_module_codes",
+    "get_module_tier",
+    "is_core_module",
+    "is_internal_module",
+    # Service
     "ModuleService",
     "module_service",
+    # Events
+    "ModuleEvent",
+    "ModuleEventData",
+    "ModuleEventBus",
+    "module_event_bus",
 ]