fix: resolve architecture validation violations

- Add invoice exceptions module with proper exception hierarchy
- Replace HTTPException with service-layer exceptions in invoice API
- Add InvoicePDFGeneratedResponse and InvoiceStatsResponse Pydantic models
- Replace db.commit() with db.flush() in services for proper transaction control
- Update invoice service to use exceptions from app/exceptions/invoice.py

All 14 errors and 14 warnings are now resolved. Validation passes with
only INFO-level findings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/exceptions/__init__.py

@@ -90,6 +90,18 @@ from .customer import (
     InvalidCustomerCredentialsException,
 )
 
+# Invoice exceptions
+from .invoice import (
+    InvoiceNotFoundException,
+    InvoicePDFGenerationException,
+    InvoicePDFNotFoundException,
+    InvoiceSettingsAlreadyExistException,
+    InvoiceSettingsNotFoundException,
+    InvoiceValidationException,
+    InvalidInvoiceStatusTransitionException,
+    OrderNotFoundException as OrderNotFoundForInvoiceException,
+)
+
 # Inventory exceptions
 from .inventory import (
     InsufficientInventoryException,
@@ -272,6 +284,15 @@ __all__ = [
     "TeamValidationException",
     "InvalidInvitationDataException",
     "InvalidInvitationTokenException",
+    # Invoice exceptions
+    "InvoiceNotFoundException",
+    "InvoiceSettingsNotFoundException",
+    "InvoiceSettingsAlreadyExistException",
+    "InvoiceValidationException",
+    "InvoicePDFGenerationException",
+    "InvoicePDFNotFoundException",
+    "InvalidInvoiceStatusTransitionException",
+    "OrderNotFoundForInvoiceException",
     # Inventory exceptions
     "InventoryNotFoundException",
     "InsufficientInventoryException",

app/exceptions/invoice.py

@@ -0,0 +1,118 @@
+# app/exceptions/invoice.py
+"""
+Invoice-related exceptions.
+
+This module provides exception classes for invoice operations including:
+- Invoice not found errors
+- Invoice settings validation
+- PDF generation errors
+- Invoice status transitions
+"""
+
+from typing import Any
+
+from .base import BusinessLogicException, ResourceNotFoundException, WizamartException
+
+
+class InvoiceNotFoundException(ResourceNotFoundException):
+    """Raised when an invoice is not found."""
+
+    def __init__(self, invoice_id: int | str):
+        super().__init__(
+            resource_type="Invoice",
+            identifier=str(invoice_id),
+            error_code="INVOICE_NOT_FOUND",
+        )
+
+
+class InvoiceSettingsNotFoundException(ResourceNotFoundException):
+    """Raised when invoice settings are not found for a vendor."""
+
+    def __init__(self, vendor_id: int):
+        super().__init__(
+            resource_type="InvoiceSettings",
+            identifier=str(vendor_id),
+            message="Invoice settings not found. Create settings first.",
+            error_code="INVOICE_SETTINGS_NOT_FOUND",
+        )
+
+
+class InvoiceSettingsAlreadyExistException(WizamartException):
+    """Raised when trying to create invoice settings that already exist."""
+
+    def __init__(self, vendor_id: int):
+        super().__init__(
+            message=f"Invoice settings already exist for vendor {vendor_id}",
+            error_code="INVOICE_SETTINGS_ALREADY_EXIST",
+            status_code=409,
+            details={"vendor_id": vendor_id},
+        )
+
+
+class InvoiceValidationException(BusinessLogicException):
+    """Raised when invoice data validation fails."""
+
+    def __init__(self, message: str, details: dict[str, Any] | None = None):
+        super().__init__(
+            message=message,
+            error_code="INVOICE_VALIDATION_ERROR",
+            details=details,
+        )
+
+
+class InvoicePDFGenerationException(WizamartException):
+    """Raised when PDF generation fails."""
+
+    def __init__(self, invoice_id: int, reason: str):
+        super().__init__(
+            message=f"Failed to generate PDF for invoice {invoice_id}: {reason}",
+            error_code="INVOICE_PDF_GENERATION_FAILED",
+            status_code=500,
+            details={"invoice_id": invoice_id, "reason": reason},
+        )
+
+
+class InvoicePDFNotFoundException(ResourceNotFoundException):
+    """Raised when invoice PDF file is not found."""
+
+    def __init__(self, invoice_id: int):
+        super().__init__(
+            resource_type="InvoicePDF",
+            identifier=str(invoice_id),
+            message="PDF file not found. Generate the PDF first.",
+            error_code="INVOICE_PDF_NOT_FOUND",
+        )
+
+
+class InvalidInvoiceStatusTransitionException(BusinessLogicException):
+    """Raised when an invalid invoice status transition is attempted."""
+
+    def __init__(
+        self,
+        current_status: str,
+        new_status: str,
+        reason: str | None = None,
+    ):
+        message = f"Cannot change invoice status from '{current_status}' to '{new_status}'"
+        if reason:
+            message += f": {reason}"
+
+        super().__init__(
+            message=message,
+            error_code="INVALID_INVOICE_STATUS_TRANSITION",
+            details={
+                "current_status": current_status,
+                "new_status": new_status,
+            },
+        )
+
+
+class OrderNotFoundException(ResourceNotFoundException):
+    """Raised when an order for invoice creation is not found."""
+
+    def __init__(self, order_id: int):
+        super().__init__(
+            resource_type="Order",
+            identifier=str(order_id),
+            error_code="ORDER_NOT_FOUND_FOR_INVOICE",
+        )