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.

go.mod

@@ -17,7 +17,21 @@ require (
 )
 
 require (
+	cel.dev/expr v0.24.0 // indirect
+	cloud.google.com/go v0.121.6 // indirect
+	cloud.google.com/go/auth v0.17.0 // indirect
+	cloud.google.com/go/auth/oauth2adapt v0.2.8 // indirect
+	cloud.google.com/go/compute/metadata v0.9.0 // indirect
+	cloud.google.com/go/iam v1.5.2 // indirect
+	cloud.google.com/go/monitoring v1.24.2 // indirect
+	cloud.google.com/go/storage v1.57.2 // indirect
 	filippo.io/edwards25519 v1.1.0 // indirect
+	github.com/Azure/azure-sdk-for-go/sdk/azcore v1.20.0 // indirect
+	github.com/Azure/azure-sdk-for-go/sdk/internal v1.11.2 // indirect
+	github.com/Azure/azure-sdk-for-go/sdk/storage/azblob v1.6.3 // indirect
+	github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.29.0 // indirect
+	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric v0.53.0 // indirect
+	github.com/GoogleCloudPlatform/opentelemetry-operations-go/internal/resourcemapping v0.53.0 // indirect
 	github.com/aws/aws-sdk-go-v2 v1.40.0 // indirect
 	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.3 // indirect
 	github.com/aws/aws-sdk-go-v2/config v1.32.2 // indirect
@@ -39,12 +53,24 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/sts v1.41.2 // indirect
 	github.com/aws/smithy-go v1.23.2 // indirect
 	github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/charmbracelet/colorprofile v0.2.3-0.20250311203215-f60798e515dc // indirect
 	github.com/charmbracelet/x/ansi v0.10.1 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.13-0.20250311204145-2c3ea96c31dd // indirect
 	github.com/charmbracelet/x/term v0.2.1 // indirect
+	github.com/cncf/xds/go v0.0.0-20250501225837-2ac532fd4443 // indirect
 	github.com/creack/pty v1.1.17 // indirect
+	github.com/envoyproxy/go-control-plane/envoy v1.32.4 // indirect
+	github.com/envoyproxy/protoc-gen-validate v1.2.1 // indirect
 	github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f // indirect
+	github.com/felixge/httpsnoop v1.0.4 // indirect
+	github.com/go-jose/go-jose/v4 v4.1.2 // indirect
+	github.com/go-logr/logr v1.4.3 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/google/s2a-go v0.1.9 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/googleapis/enterprise-certificate-proxy v0.3.7 // indirect
+	github.com/googleapis/gax-go/v2 v2.15.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
@@ -56,10 +82,31 @@ require (
 	github.com/muesli/ansi v0.0.0-20230316100256-276c6243b2f6 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/muesli/termenv v0.16.0 // indirect
+	github.com/planetscale/vtprotobuf v0.6.1-0.20240319094008-0393e58bdf10 // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
+	github.com/spiffe/go-spiffe/v2 v2.5.0 // indirect
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
-	golang.org/x/crypto v0.37.0 // indirect
-	golang.org/x/sync v0.13.0 // indirect
-	golang.org/x/sys v0.36.0 // indirect
-	golang.org/x/text v0.24.0 // indirect
+	github.com/zeebo/errs v1.4.0 // indirect
+	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
+	go.opentelemetry.io/contrib/detectors/gcp v1.36.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.61.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.61.0 // indirect
+	go.opentelemetry.io/otel v1.37.0 // indirect
+	go.opentelemetry.io/otel/metric v1.37.0 // indirect
+	go.opentelemetry.io/otel/sdk v1.37.0 // indirect
+	go.opentelemetry.io/otel/sdk/metric v1.37.0 // indirect
+	go.opentelemetry.io/otel/trace v1.37.0 // indirect
+	golang.org/x/crypto v0.43.0 // indirect
+	golang.org/x/net v0.46.0 // indirect
+	golang.org/x/oauth2 v0.33.0 // indirect
+	golang.org/x/sync v0.18.0 // indirect
+	golang.org/x/sys v0.37.0 // indirect
+	golang.org/x/text v0.30.0 // indirect
+	golang.org/x/time v0.14.0 // indirect
+	google.golang.org/api v0.256.0 // indirect
+	google.golang.org/genproto v0.0.0-20250603155806-513f23925822 // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20250818200422-3122310a409c // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251103181224-f26f9409b101 // indirect
+	google.golang.org/grpc v1.76.0 // indirect
+	google.golang.org/protobuf v1.36.10 // indirect
 )