Add secure hosted sharing for evaluation reports

## Summary

Add an explicit, secure way to publish a Flowly evaluation report at a shareable hosted URL. This is an optional follow-up to #38.

## Scope

- Publish a selected immutable report snapshot
- Provide revocable, access-controlled links with clear expiry and visibility
- Redact secrets, repository-local paths, and sensitive workload content before upload
- Preserve suite/model lineage and metric definitions in the shared view

## Acceptance criteria

- [ ] Sharing is opt-in and never uploads reports during normal local or CI evaluation
- [ ] Published snapshots are immutable, identifiable, and revocable
- [ ] Access and expiry behavior is documented and tested
- [ ] A redaction boundary prevents credentials and sensitive local data from being published
- [ ] Shared reports show enough lineage to interpret and reproduce results

## Non-goals

- Interactive dashboard exploration
- Automatic model routing
- Public-by-default reports

Start by reading issue #38 and locating the existing evaluation report and snapshot flow in the TypeScript project. Map where opt-in publishing, redaction, access control, expiry, revocation, and lineage would fit before proposing an implementation. Done means the acceptance criteria are covered by documented behavior and tests, without dashboard exploration or public-by-default reports.

Written by the indexing model from the issue text.