Threads engagement backfeeds to webmention.io via upstream Bridgy support

## Summary

Add a Threads silo to Bridgy upstream so likes, replies, and reposts
on the syndicated Threads copy backfeed to webmention.io and render on
the canonical post.

## Motivation

Split out of #201, which narrowed to Bluesky. Bridgy backfeeds
Bluesky, Mastodon, Flickr, GitHub, and Reddit; Threads has never been
supported, so the copies dlvr.it publishes there collect engagement
nothing can retrieve.

Upstream is the cheaper path and it is open. snarfed/bridgy#1507 has
tracked Threads since July 2023; in April 2024 the maintainer said he
will not build it himself but welcomes a contribution, and both
Bluesky and Reddit arrived that way. `oauth-dropins` gained Threads
support in March 2025, so the auth half already exists.

A poller built into this blog was the alternative and it loses on
cost: a registered Meta app, a token refreshing every sixty days, and
a secret in CI — the maintenance and credential burden ADR 0001 chose
dlvr.it to avoid. It also would not satisfy webmention.io, which
verifies that a source page links to the target, so it would need
hosted source pages this stack has nowhere to put. Upstream keeps the
credential with Bridgy and every Bridgy user gets the same result.

## Scope

- Confirm how a Bridgy silo splits across `granary` (platform API to
microformats2) and `bridgy` (OAuth, polling, webmention sending),
using the Bluesky and Reddit silos as precedent.
- Register a Meta app with the read scopes a silo needs, starting from
`threads_basic`.
- Implement the silo: fetch own posts, replies, likes, and reposts;
discover the canonical link in each syndicated copy.
- Open the upstream PR against `snarfed/bridgy`, referencing #1507.
- Once merged and deployed, sign in to brid.gy with Threads and
confirm backfeed reaches canonical URLs.

## Acceptance criteria

- [ ] Threads silo merged upstream and live on brid.gy.
- [ ] Bridgy connected to Threads for this site.
- [ ] A like, reply, or repost on a syndicated Threads copy backfeeds
to webmention.io and renders on the canonical post.

## Additional context

- Upstream tracking issue:
- Sibling: #201 (Bluesky half, unblocked and account-only).
- Parent: #148.
- Threads engagement metrics: