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/definition.py

@@ -0,0 +1,79 @@
+# app/modules/payments/definition.py
+"""
+Payments module definition.
+
+Defines the payments module including its features, menu items,
+and route configurations.
+
+The payments module provides gateway abstractions that can be used by:
+- billing module: For platform subscriptions and invoices
+- orders module: For customer checkout payments
+
+This separation allows:
+1. Using payments standalone (e.g., one-time payments without subscriptions)
+2. Billing without orders (platform subscription only)
+3. Orders without billing (customer payments only)
+"""
+
+from app.modules.base import ModuleDefinition
+from models.database.admin_menu_config import FrontendType
+
+
+def _get_admin_router():
+    """Lazy import of admin router to avoid circular imports."""
+    from app.modules.payments.routes.admin import admin_router
+
+    return admin_router
+
+
+def _get_vendor_router():
+    """Lazy import of vendor router to avoid circular imports."""
+    from app.modules.payments.routes.vendor import vendor_router
+
+    return vendor_router
+
+
+# Payments module definition
+payments_module = ModuleDefinition(
+    code="payments",
+    name="Payment Gateways",
+    description=(
+        "Payment gateway integrations for Stripe, PayPal, and bank transfers. "
+        "Provides payment processing, refunds, and payment method management."
+    ),
+    version="1.0.0",
+    features=[
+        "payment_processing",  # Process payments
+        "payment_refunds",  # Issue refunds
+        "payment_methods",  # Store payment methods
+        "stripe_gateway",  # Stripe integration
+        "paypal_gateway",  # PayPal integration
+        "bank_transfer",  # Bank transfer support
+        "transaction_history",  # Transaction records
+    ],
+    menu_items={
+        FrontendType.ADMIN: [
+            "payment-gateways",  # Configure payment gateways
+        ],
+        FrontendType.VENDOR: [
+            "payment-methods",  # Manage stored payment methods
+        ],
+    },
+    is_core=False,
+    is_internal=False,
+)
+
+
+def get_payments_module_with_routers() -> ModuleDefinition:
+    """
+    Get payments module with routers attached.
+
+    This function attaches the routers lazily to avoid circular imports
+    during module initialization.
+    """
+    payments_module.admin_router = _get_admin_router()
+    payments_module.vendor_router = _get_vendor_router()
+    return payments_module
+
+
+__all__ = ["payments_module", "get_payments_module_with_routers"]