Add integration tests for deduplication and ref_count

- Add test_integration_uploads.py with 12 tests for duplicate upload scenarios
- Add test_ref_count.py with 7 tests for ref_count management
- Fix ArtifactDetailResponse to include sha256 and checksum fields
- Fix health check SQL warning by wrapping in text()
- Update tests to use unique content per test run for idempotency

backend/pytest.ini

@@ -4,7 +4,7 @@ python_files = test_*.py
 python_functions = test_*
 python_classes = Test*
 asyncio_mode = auto
-addopts = -v --tb=short
+addopts = -v --tb=short --cov=app --cov-report=term-missing --cov-report=html:coverage_html --cov-fail-under=0
 filterwarnings =
     ignore::DeprecationWarning
     ignore::UserWarning
@@ -12,3 +12,18 @@ markers =
     unit: Unit tests (no external dependencies)
     integration: Integration tests (require database/storage)
     slow: Slow tests (skip with -m "not slow")
+
+# Coverage configuration
+[coverage:run]
+source = app
+omit = 
+    */tests/*
+    */__pycache__/*
+
+[coverage:report]
+exclude_lines =
+    pragma: no cover
+    def __repr__
+    raise NotImplementedError
+    if __name__ == .__main__.:
+    pass