feat: add experimental TUS protocol files (not active)

Commit notes

Added TUS protocol implementation files for future reference: - backend/src/routes/tus.ts - TUS server route (needs tusd or manual impl) - frontend/src/lib/upload/tus-client.ts - TUS client using tus-js-client

These are NOT active - the custom upload system is used instead.

TUS vs Custom comparison documented in the files.

The custom system provides: - Direct S3 presigned URL uploads (faster, no server bottleneck) - IndexedDB persistence (cross-session resume on same device) - Native Bun/Elysia compatibility - 8+ concurrent chunk uploads

TUS would provide: - Cross-device resume (server-side state) - Protocol standardization - Battle-tested implementation

For TUS, the recommended approach is running tusd as a microservice.

Co-authored-by: armin.naimi <[email redacted]>