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",