Add storage abstraction, stats endpoints, garbage collection, and test infrastructure

- Add StorageBackend protocol for backend-agnostic storage interface
- Add health check with storage and database connectivity verification
- Add garbage collection endpoints for orphaned artifacts (ref_count=0)
- Add deduplication statistics endpoints (/api/v1/stats, /stats/storage, /stats/deduplication)
- Add per-project statistics endpoint
- Add verify_integrity method for post-upload hash validation
- Set up pytest infrastructure with mock S3 client
- Add unit tests for hash calculation and duplicate detection

backend/pytest.ini

@@ -0,0 +1,14 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_functions = test_*
+python_classes = Test*
+asyncio_mode = auto
+addopts = -v --tb=short
+filterwarnings =
+    ignore::DeprecationWarning
+    ignore::UserWarning
+markers =
+    unit: Unit tests (no external dependencies)
+    integration: Integration tests (require database/storage)
+    slow: Slow tests (skip with -m "not slow")