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

.sisyphus/plans/zugferd-service.md

@@ -829,7 +829,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ### Wave 3: Validation Logic
 
-- [ ] 7. Validator Implementation (TDD)
+- [x] 7. Validator Implementation (TDD)
 
   **What to do**:
   - Write tests first for each validation check
@@ -965,7 +965,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ### Wave 3 (continued): API Foundation
 
-- [ ] 8. FastAPI Application Structure
+- [x] 8. FastAPI Application Structure
 
   **What to do**:
   - Create FastAPI app instance in main.py
@@ -1018,7 +1018,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ---
 
-- [ ] 9. Health Endpoint Implementation
+- [x] 9. Health Endpoint Implementation
 
   **What to do**:
   - Implement `GET /health` endpoint