feat: add self-contained structure to remaining modules

Add exceptions, models, schemas, services directories to modules:

customers:
- exceptions.py, models/, schemas/, services/

inventory:
- exceptions.py, models/, schemas/, services/

messaging:
- exceptions.py, models/, schemas/, services/

monitoring:
- exceptions.py, models/, schemas/, services/

orders:
- exceptions.py, models/, schemas/, services/

payments:
- Updated __init__.py

All modules now have the standard self-contained directory
structure ready for future migration of business logic.

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

app/modules/monitoring/models/__init__.py

@@ -0,0 +1,21 @@
+# app/modules/monitoring/models/__init__.py
+"""
+Monitoring module database models.
+
+Re-exports monitoring-related models from their source locations.
+"""
+
+# CapacitySnapshot is in billing module (tracks system capacity over time)
+from app.modules.billing.models import CapacitySnapshot
+
+# Admin notification and logging models
+from models.database.admin import (
+    AdminNotification,
+    PlatformAlert,
+)
+
+__all__ = [
+    "CapacitySnapshot",
+    "AdminNotification",
+    "PlatformAlert",
+]