feat(api): add validator, FastAPI app structure, and health endpoint

Wave 3 tasks complete:
- Task 7: Validator with 4 checks (pflichtfelder, betraege, ustid, pdf_abgleich)
- Task 8: FastAPI app with CORS, exception handlers, JSON logging
- Task 9: Health endpoint returning status and version

Features:
- validate_invoice() runs selected validation checks
- Exception handlers for ExtractionError and generic errors
- GET /health returns {status: healthy, version: 1.0.0}

Tests: 52 validator tests covering all validation rules

src/main.py

@@ -1,7 +1,37 @@
 """FastAPI application for ZUGFeRD invoice processing."""
 
+import json
+import logging
+from datetime import datetime
+
 import uvicorn
-from fastapi import FastAPI
+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
+
+
+class JSONFormatter(logging.Formatter):
+    def format(self, record):
+        log_data = {
+            "timestamp": datetime.utcnow().isoformat() + "Z",
+            "level": record.levelname,
+            "message": record.getMessage(),
+        }
+        if hasattr(record, "data"):
+            log_data["data"] = record.data
+        return json.dumps(log_data)
+
+
+logger = logging.getLogger(__name__)
+if not logger.handlers:
+    handler = logging.StreamHandler()
+    handler.setFormatter(JSONFormatter())
+    logger.addHandler(handler)
+    logger.setLevel(logging.INFO)
+
 
 app = FastAPI(
     title="ZUGFeRD Service",
@@ -9,6 +39,48 @@ app = FastAPI(
     description="REST API for ZUGFeRD invoice extraction and validation",
 )
 
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.exception_handler(ExtractionError)
+async def extraction_error_handler(request: Request, exc: ExtractionError):
+    return JSONResponse(
+        status_code=400,
+        content={
+            "error": exc.error_code,
+            "message": exc.message,
+            "details": exc.details,
+        },
+    )
+
+
+@app.exception_handler(Exception)
+async def generic_error_handler(request: Request, exc: Exception):
+    logger.error(f"Internal error: {exc}")
+    return JSONResponse(
+        status_code=500,
+        content={
+            "error": "internal_error",
+            "message": "An internal error occurred",
+        },
+    )
+
+
+@app.get("/health", response_model=HealthResponse)
+async def health_check() -> HealthResponse:
+    """Health check endpoint.
+
+    Returns:
+        HealthResponse with status and version.
+    """
+    return HealthResponse(status="healthy", version="1.0.0")
+
 
 def run(host: str = "0.0.0.0", port: int = 5000) -> None:
     """Run the FastAPI application.