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/admin.py

@@ -0,0 +1,44 @@
+# app/modules/payments/routes/admin.py
+"""
+Admin routes for payments module.
+
+Provides routes for:
+- Payment gateway configuration
+- Transaction monitoring
+- Refund management
+"""
+
+from fastapi import APIRouter
+
+admin_router = APIRouter(prefix="/payments", tags=["Payments (Admin)"])
+
+
+@admin_router.get("/gateways")
+async def list_gateways():
+    """List configured payment gateways."""
+    # TODO: Implement gateway listing
+    return {
+        "gateways": [
+            {"code": "stripe", "name": "Stripe", "enabled": True},
+            {"code": "paypal", "name": "PayPal", "enabled": False},
+            {"code": "bank_transfer", "name": "Bank Transfer", "enabled": True},
+        ]
+    }
+
+
+@admin_router.get("/transactions")
+async def list_transactions():
+    """List recent transactions across all gateways."""
+    # TODO: Implement transaction listing
+    return {"transactions": [], "total": 0}
+
+
+@admin_router.post("/refunds/{transaction_id}")
+async def issue_refund(transaction_id: str, amount: float | None = None):
+    """Issue a refund for a transaction."""
+    # TODO: Implement refund logic
+    return {
+        "status": "pending",
+        "transaction_id": transaction_id,
+        "refund_amount": amount,
+    }