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/main.py

@@ -1,5 +1,6 @@
 """FastAPI application for ZUGFeRD invoice processing."""
 
+import base64
 import json
 import logging
 from datetime import datetime
@@ -9,8 +10,15 @@ from fastapi import FastAPI, HTTPException, Request
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 
-from src.extractor import ExtractionError
-from src.models import HealthResponse
+from src.extractor import ExtractionError, extract_zugferd
+from src.models import (
+    ExtractRequest,
+    ExtractResponse,
+    HealthResponse,
+    ValidateRequest,
+    ValidateResponse,
+)
+from src.validator import validate_invoice
 
 
 class JSONFormatter(logging.Formatter):
@@ -60,6 +68,25 @@ async def extraction_error_handler(request: Request, exc: ExtractionError):
     )
 
 
+@app.exception_handler(HTTPException)
+async def http_exception_handler(request: Request, exc: HTTPException):
+    if isinstance(exc.detail, dict) and "error" in exc.detail:
+        return JSONResponse(
+            status_code=exc.status_code,
+            content={
+                "error": exc.detail.get("error"),
+                "message": exc.detail.get("message"),
+            },
+        )
+    return JSONResponse(
+        status_code=exc.status_code,
+        content={
+            "error": "http_error",
+            "message": str(exc.detail),
+        },
+    )
+
+
 @app.exception_handler(Exception)
 async def generic_error_handler(request: Request, exc: Exception):
     logger.error(f"Internal error: {exc}")
@@ -82,6 +109,41 @@ async def health_check() -> HealthResponse:
     return HealthResponse(status="healthy", version="1.0.0")
 
 
+@app.post("/extract", response_model=ExtractResponse)
+async def extract_pdf(request: ExtractRequest) -> ExtractResponse:
+    """Extract ZUGFeRD data from PDF.
+
+    Args:
+        request: ExtractRequest with pdf_base64 field
+
+    Returns:
+        ExtractResponse with extraction results
+    """
+    try:
+        pdf_bytes = base64.b64decode(request.pdf_base64)
+    except Exception:
+        raise HTTPException(
+            status_code=400,
+            detail={"error": "invalid_base64", "message": "Invalid base64 encoding"},
+        )
+
+    return extract_zugferd(pdf_bytes)
+
+
+@app.post("/validate", response_model=ValidateResponse)
+async def validate_invoice_endpoint(request: ValidateRequest) -> ValidateResponse:
+    """Validate ZUGFeRD invoice data.
+
+    Args:
+        request: ValidateRequest with xml_data, pdf_text, checks
+
+    Returns:
+        ValidateResponse with validation results
+    """
+    result = validate_invoice(request)
+    return ValidateResponse(result=result)
+
+
 def run(host: str = "0.0.0.0", port: int = 5000) -> None:
     """Run the FastAPI application.