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/services/invoice_pdf_service.py

@@ -97,7 +97,7 @@ class InvoicePDFService:
         # Update invoice record with PDF path and timestamp
         invoice.pdf_path = str(pdf_path)
         invoice.pdf_generated_at = datetime.now(UTC)
-        db.commit()
+        db.flush()
 
         return str(pdf_path)
 
@@ -151,7 +151,7 @@ class InvoicePDFService:
         # Clear PDF fields
         invoice.pdf_path = None
         invoice.pdf_generated_at = None
-        db.commit()
+        db.flush()
 
         return True