feat: add S3 and Azure Blob storage backend test pages

[tomdesair]

Summary

This PR adds two new dedicated test pages and upload endpoints for S3 Object Storage and Azure Blob Storage alongside the existing Disk Storage backend in the Spring Boot demo application:

• http://localhost:8080/test/: Default Disk storage backend (DiskStorageService + LeaseFileLockingService) targeting /test/api/upload
• http://localhost:8080/test-s3/: S3-compatible storage backend (S3StorageService + S3LockingService via MinIO Java SDK) targeting /test-s3/api/upload
• http://localhost:8080/test-azure/: Azure Blob storage backend (AzureBlobStorageService + AzureBlobLockingService via Azure SDK) targeting /test-azure/api/upload
• http://localhost:8080/: Clean root landing page with navigation links to all three storage backend test pages.

Key Changes

• Properties: Configured and documented settings for Disk, S3 (tus.s3.*), and Azure Blob (tus.azure.*) in application.properties.
• Services: Instantiated 3 separate TusFileUploadService beans in App.java.
• Controller: Mapped upload endpoints for /test/api/upload, /test-s3/api/upload, and /test-azure/api/upload in FileUploadController.java.
• Tomcat Valve: Updated TusInterimResponseTomcatValve.java to support multi-service HTTP 104 interim response emission.
• Frontend: Updated uppy-fileupload.js to dynamically resolve data-endpoint from .DashboardContainer.
• Documentation: Updated README.md with detailed backend testing instructions (Docker MinIO & Azurite) and a Production Considerations section.
• Testing: Added MockMvc tests covering all routes and endpoints in FileUploadControllerTest and TusInterimResponseTomcatValveTest.