Web dashboard for key & upload status

[ServerSideHannes]

Summary

Add a lightweight web UI for monitoring encryption key status, active uploads, and proxy health.

Problem

Operators currently have no visual way to see:

• Which encryption key is active
• How many multipart uploads are in progress
• Memory pressure and concurrency status
• Recent errors or rejected requests

Prometheus + Grafana (#9) covers metrics, but a built-in dashboard provides a quick overview without external tooling.

Proposal

A simple web UI served at /admin (or configurable path), showing:

Key status:

• Active KEK fingerprint (hash, not the key itself)
• Number of objects encrypted with each key version (if key rotation is implemented)

Upload status:

• Active multipart uploads (count, oldest, bucket/key)
• Upload completion rate
• Failed/aborted uploads

System health:

• Memory budget usage (reserved / limit)
• Active concurrent requests
• Recent 503 rejections
• Uptime and version

Tech stack:

• Lightweight: server-rendered HTML or minimal JS (no React/Vue)
• No additional dependencies if possible (Jinja2 is already available via FastAPI)
• Protected by auth (basic auth or same S3 credentials)

Considerations

• Must not expose encryption keys or sensitive data
• Should be disabled by default (S3PROXY_ADMIN_UI=true)
• Read-only: no mutations from the UI
• Keep it minimal — this is an ops tool, not a full management console