fix: resolve test failures and update flake.nix for factur-x source distribution

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

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

.sisyphus/plans/zugferd-service.md

@@ -70,12 +70,12 @@ Create a production-ready, stateless REST API that extracts ZUGFeRD/Factur-X inv
 - `README.md` - Installation and usage documentation
 
 ### Definition of Done
-- [ ] `nix build .#zugferd-service` completes without errors
-- [ ] `docker build -t zugferd-service .` produces image <500MB
-- [ ] `pytest` runs all tests with 100% pass rate
-- [ ] `curl http://localhost:5000/health` returns `{"status": "healthy", "version": "1.0.0"}`
-- [ ] All ZUGFeRD profiles correctly detected from sample PDFs
-- [ ] All validation checks produce expected errors/warnings
+- [x] `nix build .#zugferd-service` completes without errors
+- [x] `docker build -t zugferd-service .` produces image <500MB
+- [x] `pytest` runs all tests with 100% pass rate
+- [x] `curl http://localhost:5000/health` returns `{"status": "healthy", "version": "1.0.0"}`
+- [x] All ZUGFeRD profiles correctly detected from sample PDFs
+- [x] All validation checks produce expected errors/warnings
 
 ### Must Have
 - All 3 API endpoints as specified
@@ -1452,7 +1452,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ### Wave 6: Nix Packaging
 
-- [ ] 16. Nix Flake Packaging
+- [x] 16. Nix Flake Packaging
 
   **What to do**:
   - Create flake.nix with buildPythonApplication
@@ -1589,7 +1589,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ---
 
-- [ ] 17. NixOS Service Module Example
+- [x] 17. NixOS Service Module Example
 
   **What to do**:
   - Create example NixOS module for deployment
@@ -1635,7 +1635,7 @@ Critical Path: Task 1 → Task 4 → Task 7 → Task 10 → Task 13 → Task 16
 
 ---
 
-- [ ] 18. README Documentation
+- [x] 18. README Documentation
 
   **What to do**:
   - Create comprehensive README.md
@@ -1753,12 +1753,12 @@ curl -X POST http://localhost:5000/extract \
 ```
 
 ### Final Checklist
-- [ ] All 18 tasks completed
-- [ ] All tests pass (pytest)
-- [ ] Docker image builds (<500MB)
-- [ ] Docker container runs and responds
-- [ ] Nix flake builds without errors
-- [ ] Nix package runs and responds
-- [ ] All endpoints return expected responses
-- [ ] README documents all features
-- [ ] No "Must NOT Have" items present
+- [x] All 18 tasks completed
+- [x] All tests pass (pytest)
+- [x] Docker image builds (<500MB)
+- [x] Docker container runs and responds
+- [x] Nix flake builds without errors
+- [x] Nix package runs and responds
+- [x] All endpoints return expected responses
+- [x] README documents all features
+- [x] No "Must NOT Have" items present