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>

models/schema/invoice.py

@@ -281,3 +281,30 @@ class InvoiceListPaginatedResponse(BaseModel):
     page: int
     per_page: int
     pages: int
+
+
+# ============================================================================
+# PDF Response
+# ============================================================================
+
+
+class InvoicePDFGeneratedResponse(BaseModel):
+    """Response for PDF generation."""
+
+    pdf_path: str
+    message: str = "PDF generated successfully"
+
+
+class InvoiceStatsResponse(BaseModel):
+    """Invoice statistics response."""
+
+    total_invoices: int
+    total_revenue_cents: int
+    draft_count: int
+    issued_count: int
+    paid_count: int
+    cancelled_count: int
+
+    @property
+    def total_revenue(self) -> float:
+        return self.total_revenue_cents / 100