refactor: migrate modules from re-exports to canonical implementations

Move actual code implementations into module directories:
- orders: 5 services, 4 models, order/invoice schemas
- inventory: 3 services, 2 models, 30+ schemas
- customers: 3 services, 2 models, customer schemas
- messaging: 3 services, 2 models, message/notification schemas
- monitoring: background_tasks_service
- marketplace: 5+ services including letzshop submodule
- dev_tools: code_quality_service, test_runner_service
- billing: billing_service
- contracts: definition.py

Legacy files in app/services/, models/database/, models/schema/
now re-export from canonical module locations for backwards
compatibility. Architecture validator passes with 0 errors.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/contracts/definition.py

@@ -0,0 +1,24 @@
+# app/modules/contracts/definition.py
+"""
+Contracts module definition.
+
+Cross-module contracts and Protocol interfaces.
+Infrastructure module - cannot be disabled.
+"""
+
+from app.modules.base import ModuleDefinition
+
+contracts_module = ModuleDefinition(
+    code="contracts",
+    name="Module Contracts",
+    description="Cross-module contracts using Protocol pattern for type-safe inter-module communication.",
+    version="1.0.0",
+    is_core=True,
+    features=[
+        "service_protocols",
+        "cross_module_interfaces",
+    ],
+    menu_items={},  # Infrastructure module - no UI
+)
+
+__all__ = ["contracts_module"]