feat(api): implement /extract and /validate endpoints with error handling

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/validator.py

@@ -257,11 +257,26 @@ def validate_invoice(request: ValidateRequest) -> ValidationResult:
     all_errors = []
     all_warnings = []
 
-    xml_data = XmlData(**request.xml_data)
-
     checks_run = 0
     checks_passed = 0
 
+    if not request.checks:
+        return ValidationResult(
+            is_valid=True,
+            errors=[],
+            warnings=[],
+            summary={
+                "total_checks": 0,
+                "checks_passed": 0,
+                "checks_failed": 0,
+                "critical_errors": 0,
+                "warnings": 0,
+            },
+            validation_time_ms=0,
+        )
+
+    xml_data = XmlData(**request.xml_data)
+
     # Run requested checks
     for check_name in request.checks:
         check_errors: list[ErrorDetail] = []