Populate item.meta in the resolvers so {postId} and {username} work

[jamditis]

#47 landed the filename template engine. {platform}, {type}, {index} and {date} work today. {postId} and {username} render as nothing, because no resolver populates item.meta yet.

The renderer absorbs a missing token along with its separator, so a template naming them degrades to a shorter name rather than a broken one — but two of six advertised tokens being permanently empty is not a finished feature.

Contract

Resolvers attach the structured parts alongside the name they already build:

items.push({
  url,
  type: 'image',
  filename: id ? `photo_${id}_${index}` : null,  // unchanged, still the default
  meta: { postId: id, username },
});

filename stays as-is. It is what an unconfigured user gets, and #47 keeps templates opt-in precisely so an update does not rename anyone's files.

Platforms

• facebook.js — do this one after Stop the Facebook album walk from emitting one photo twice #45 merges. It rewrites the same item-construction lines, so doing both at once conflicts.
• instagram.js — has shortcode in scope.
• twitter.js — has tweetId.
• linkedin.js — has the post id.
• tiktok.js — has videoId.
• bluesky.js — check what it exposes.

username is only available where the resolver can read it from the DOM or the API response. Leave it unset where it cannot rather than guessing: a wrong username in a filename is worse than a missing one.

Do not set postId on the facebook capture fallback. That store is page-wide and those URLs were not read from the post's own DOM, so an id extracted from one is not reliably this post's.