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/exceptions.py

@@ -0,0 +1,50 @@
+# app/modules/monitoring/exceptions.py
+"""
+Monitoring module exceptions.
+
+Module-specific exceptions for monitoring functionality.
+"""
+
+from app.exceptions.base import (
+    BusinessLogicException,
+    ResourceNotFoundException,
+)
+
+
+class TaskNotFoundException(ResourceNotFoundException):
+    """Raised when a background task is not found."""
+
+    def __init__(self, task_id: str):
+        super().__init__(
+            resource_type="BackgroundTask",
+            identifier=task_id,
+            error_code="TASK_NOT_FOUND",
+        )
+
+
+class CapacitySnapshotNotFoundException(ResourceNotFoundException):
+    """Raised when a capacity snapshot is not found."""
+
+    def __init__(self, snapshot_id: int):
+        super().__init__(
+            resource_type="CapacitySnapshot",
+            identifier=str(snapshot_id),
+            error_code="CAPACITY_SNAPSHOT_NOT_FOUND",
+        )
+
+
+class MonitoringServiceException(BusinessLogicException):
+    """Raised when a monitoring operation fails."""
+
+    def __init__(self, operation: str, reason: str):
+        super().__init__(
+            message=f"Monitoring operation '{operation}' failed: {reason}",
+            error_code="MONITORING_OPERATION_FAILED",
+        )
+
+
+__all__ = [
+    "TaskNotFoundException",
+    "CapacitySnapshotNotFoundException",
+    "MonitoringServiceException",
+]