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/requirements.txt

@@ -9,3 +9,10 @@ pydantic==2.5.3
 pydantic-settings==2.1.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
+
+# Test dependencies
+pytest>=7.4.0
+pytest-asyncio>=0.21.0
+pytest-cov>=4.1.0
+httpx>=0.25.0
+moto[s3]>=4.2.0