Bridgy backfeeds Bluesky engagement to webmention.io

## Summary

Connect [Bridgy](https://brid.gy/) so likes, reposts, and replies on
the syndicated Bluesky copies backfeed to webmention.io as webmentions,
surfacing on the canonical post.

## Motivation

Split out of #148. The receiver shipped in PR #150 (closing #144) and
the webmention.io account exists, so no engagement arrives until
Bridgy bridges the syndicated copies. #77 has shipped, so this is
unblocked and is now account configuration only.

Threads moved to #514. Bridgy has never supported it, so that half is
an upstream contribution rather than a sign-in, on its own timeline.

## Scope

- Sign in to brid.gy with Bluesky.
- Confirm Bridgy backfeeds to the canonical URLs, not just publishes.

## Acceptance criteria

- [ ] Bridgy connected to Bluesky.
- [ ] A like, repost, or reply on a syndicated Bluesky copy backfeeds
to webmention.io and renders on the canonical post.

## Additional context

Sub-task of #148. Receiver code landed in #150 (#144). Threads tracked
in #514. First-party reply paths — the gap backfeed does not
address — tracked in #515.