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.

docker-compose.gcs.yml

@@ -0,0 +1,59 @@
+version: '3.8'
+
+services:
+  # fake-gcs-server - Google Cloud Storage Emulator
+  gcs-emulator:
+    image: fsouza/fake-gcs-server:latest
+    container_name: dbbackup-gcs
+    ports:
+      - "4443:4443"
+    command: -scheme http -public-host localhost:4443 -external-url http://localhost:4443
+    healthcheck:
+      test: ["CMD", "wget", "--spider", "-q", "http://localhost:4443/storage/v1/b"]
+      interval: 5s
+      timeout: 3s
+      retries: 30
+    networks:
+      - dbbackup-net
+
+  # PostgreSQL 16 for testing
+  postgres:
+    image: postgres:16-alpine
+    container_name: dbbackup-postgres-gcs
+    environment:
+      POSTGRES_USER: testuser
+      POSTGRES_PASSWORD: testpass
+      POSTGRES_DB: testdb
+    ports:
+      - "5435:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U testuser -d testdb"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+    networks:
+      - dbbackup-net
+
+  # MySQL 8.0 for testing
+  mysql:
+    image: mysql:8.0
+    container_name: dbbackup-mysql-gcs
+    environment:
+      MYSQL_ROOT_PASSWORD: rootpass
+      MYSQL_DATABASE: testdb
+      MYSQL_USER: testuser
+      MYSQL_PASSWORD: testpass
+    ports:
+      - "3309:3306"
+    command: --default-authentication-plugin=mysql_native_password
+    healthcheck:
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "root", "-prootpass"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+    networks:
+      - dbbackup-net
+
+networks:
+  dbbackup-net:
+    driver: bridge