feat(viewer): add optional GCS and S3 cloud sources for local computer viewer

[jonasjancarik]

Summary

• keep existing local TeslaUSB viewer flow intact
• add an additive Cloud source in Viewer (Local/Cloud switch)
• add cloudviewer-api service for GCS listing/signing/streaming + local compatibility endpoints
• add local computer runtime via Docker Compose and nginx proxy wiring
• document setup in doc/CloudViewerGCS.md

Details

• frontend source selector and source-aware loading/playback in teslausb-www/html/index.html
• cloud endpoints:
  • GET /api/v1/cloud/health
  • GET /api/v1/cloud/videolist
  • GET /api/v1/cloud/object-url
  • GET /api/v1/cloud/stream
• local compatibility endpoints:
  • GET /api/v1/local/config
  • GET /api/v1/local/status
  • GET /api/v1/local/videolist
• docker files:
  • docker-compose.yml
  • docker/nginx-cloudviewer.conf
• env/docs:
  • .env.example
  • doc/CloudViewerGCS.md

Validation

• cd cloudviewer-api && go test ./...
• docker compose config
• docker compose build
• smoke:
  • GET /api/v1/cloud/videolist
  • GET /api/v1/cloud/object-url?...

Transparency: Coded by Codex; manually tested by @jonasjancarik.