feat: Sprint 4 - Azure Blob Storage and Google Cloud Storage support

Implemented full native support for Azure Blob Storage and Google Cloud Storage:

**Azure Blob Storage (internal/cloud/azure.go):**
- Native Azure SDK integration (github.com/Azure/azure-sdk-for-go)
- Block blob upload for large files (>256MB with 100MB blocks)
- Azurite emulator support for local testing
- Production Azure authentication (account name + key)
- SHA-256 integrity verification with metadata
- Streaming uploads with progress tracking

**Google Cloud Storage (internal/cloud/gcs.go):**
- Native GCS SDK integration (cloud.google.com/go/storage)
- Chunked upload for large files (16MB chunks)
- fake-gcs-server emulator support for local testing
- Application Default Credentials support
- Service account JSON key file support
- SHA-256 integrity verification with metadata
- Streaming uploads with progress tracking

**Backend Integration:**
- Updated NewBackend() factory to support azure/azblob and gs/gcs providers
- Added Name() methods to both backends
- Fixed ProgressReader usage across all backends
- Updated Config comments to document Azure/GCS support

**Testing Infrastructure:**
- docker-compose.azurite.yml: Azurite + PostgreSQL + MySQL test environment
- docker-compose.gcs.yml: fake-gcs-server + PostgreSQL + MySQL test environment
- scripts/test_azure_storage.sh: 8 comprehensive Azure integration tests
- scripts/test_gcs_storage.sh: 8 comprehensive GCS integration tests
- Both test scripts validate upload/download/verify/cleanup/restore operations

**Documentation:**
- AZURE.md: Complete guide (600+ lines) covering setup, authentication, usage
- GCS.md: Complete guide (600+ lines) covering setup, authentication, usage
- Updated CLOUD.md with Azure and GCS sections
- Updated internal/config/config.go with Azure/GCS field documentation

**Test Coverage:**
- Large file uploads (300MB for Azure, 200MB for GCS)
- Block/chunked upload verification
- Backup verification with SHA-256 checksums
- Restore from cloud URIs
- Cleanup and retention policies
- Emulator support for both providers

**Dependencies Added:**
- Azure: github.com/Azure/azure-sdk-for-go/sdk/storage/azblob v1.6.3
- GCS: cloud.google.com/go/storage v1.57.2
- Plus transitive dependencies (~50+ packages)

**Build:**
- Compiles successfully: 68MB binary
- All imports resolved
- No compilation errors

Sprint 4 closes the multi-cloud gap identified in Sprint 3 evaluation.
Users can now use Azure and GCS URIs that were previously parsed but unsupported.

internal/cloud/interface.go

@@ -79,8 +79,12 @@ func NewBackend(cfg *Config) (Backend, error) {
 			return nil, fmt.Errorf("endpoint required for Backblaze B2")
 		}
 		return NewS3Backend(cfg)
+	case "azure", "azblob":
+		return NewAzureBackend(cfg)
+	case "gs", "gcs", "google":
+		return NewGCSBackend(cfg)
 	default:
-		return nil, fmt.Errorf("unsupported cloud provider: %s (supported: s3, minio, b2)", cfg.Provider)
+		return nil, fmt.Errorf("unsupported cloud provider: %s (supported: s3, minio, b2, azure, gcs)", cfg.Provider)
 	}
 }