feat: SSE progress streaming + structured logging (feature parity)

[sktbrd]

Feature Parity PR

This brings the Oracle worker to 100% feature parity with the Mac Mini worker.

New Endpoints

Endpoint	Method	Description
/progress/:requestId	GET	Real-time SSE progress streaming
/logs?limit=10	GET	Recent transcode logs (JSON)
/stats	GET	Transcode statistics

New File

• src/logger.js — Structured logging with file persistence (logs/transcode-{hostname}.log)

SSE Progress Flow

Client subscribes: GET /progress/{correlationId}
  ↓
Client uploads: POST /transcode (with correlationId in form data)
  ↓
Server streams progress:
  5%  → receiving
  10% → transcoding (FFmpeg starts)
  0-80% → transcoding (FFmpeg real-time progress via stderr)
  85% → uploading (Pinata)
  85-100% → uploading (Pinata onUploadProgress)
  100% → complete

Why This Matters

• Webapp progress bar now works on ALL servers (not just Mac Mini)
• No more 404 errors on /progress endpoint
• Dashboard monitoring via /logs and /stats
• Structured logs for debugging failed transcodes

Backward Compatible

• All new features are additive
• Existing /transcode and /healthz endpoints unchanged
• No breaking changes to request/response format