feat: add getSyndicationUrl hook to syndicator interface

[aciccarello]

Closes #860

Summary

Adds an optional getSyndicationUrl(publication) hook to the syndicator plugin interface, enabling syndication URLs to be written into post files at creation time — without requiring a database or async syndication step.

This is needed for webmention-based syndication targets where the syndication URL is known at post creation time, but mp-syndicate-to is stripped before the file is written (correct per the Micropub spec — it is a server command, not a content property).

Changes

endpoint-syndicate

• syndicate() is now optional on syndicator targets. Targets without it are skipped rather than throwing. This allows syndicators that only implement getSyndicationUrl() to register without providing an empty no-op.

endpoint-micropub

• In post-content.js create(), after resolving syndication targets and before getPostTemplateProperties() strips mp-* keys, calls getSyndicationUrl(publication) on any target whose info.uid is in mp-syndicate-to.
• Returned URLs are normalized to an array, deduplicated against existing syndication values (exact string match), and appended to properties.syndication.
• Errors from getSyndicationUrl() are caught and logged via debug() — the create request is never failed.

Interface

// Optional method to add to a syndicator target
async getSyndicationUrl(publication) {
  // return string | string[] | null | undefined
}

A syndicator implementing this hook can return the syndication URL synchronously at post creation time. The endpoint handles appending it to syndication before the file is written.

This is scoped to create() only. Update behaviour is a separate concern.

[rmdes]

The hook solves the ordering problem in the right place — running it in create() before the template renders is what makes the URL available to the file, which is the part that can't be fixed afterwards.

One thing worth surfacing, because this PR works around it without naming it. getSyndicationTarget matches by URL origin:

const targetOrigin = new URL(target.info.uid).origin;
const syndicateToOrigin = new URL(syndicateTo).origin;
return targetOrigin === syndicateToOrigin;

IndieNews registers one target per language on a single origin, so those collapse into one. With en and fr targets registered:

mp-syndicate-to  https://news.indieweb.org/fr   ->  IndieNews (en)

This PR matches info.uid exactly instead, which gets it right — but the codebase now has two matching rules for the same concept. Worth deciding which is intended, since the origin-based one is wrong for any syndicator registering several targets on one host. My own IndieNews syndicator has the same shape, so it hits this today.

Smaller point: getSyndicationUrl failures are caught and sent to debug(). For a hook returning a constant that's fine, but as a general interface a failure means the URL silently isn't written, with nothing to indicate why.