feat: add file upload endpoint (BRIC-12)

[Kaiohz]

Summary

• Add PUT object support to StoragePort + MinioAdapter via new put_object() method
• Add POST /api/v1/files/upload endpoint accepting multipart form data (file + prefix)
• Security: file type allowlist, filename sanitization, prefix path traversal validation, 50 MB size limit
• Add UploadFileUseCase following hexagonal architecture
• Update README with upload endpoint documentation

Test plan

• 5 unit tests for UploadFileUseCase
• 6 unit tests for upload route
• 164 tests pass (0 failures)
• Manual curl testing against Docker

DO NOT MERGE (user request)

Closes BRIC-12 (backend)