feat: capture coauthors and per-item width/height/tagged_users

Two additions to the metadata sidecars, following up on the 2026-09-01
collab/dedup investigation already documented here:

- coauthors added to the post-level JSON's include list -- a direct,
  native signal for collab detection instead of inferring it from the
  filename/directory identity mismatch also documented that day.
- width, height, width_original, height_original and tagged_users get
  their own per-file sidecar (event: "file", one JSON per downloaded
  media item, named "<filename>.json"), since a carousel's items can
  each have different dimensions and tags -- data the existing
  post-level JSON has no way to represent. owner is deliberately left
  out, same reasoning as audio_user's existing exclusion.

Verified against a live re-fetch of an already-archived carousel:
correct per-item data came back, zero media re-downloaded (skip-archive
still applies; only the new sidecars are new files). 3 new tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011qAds5qr7nZRq5R4yAuxUk

docs/gallery-dl.md

@@ -301,16 +301,31 @@ participant's page shows the post, not just the one whose directory JD2/
 gallery-dl happened to land the JSON in. See that repo's `NOTES.md`,
 2026-09-01 entries, for the full fix.
 
-**`coauthors` is a native, richer signal for this that we don't currently
-capture.** The raw API response carries a `coauthors` array
-(`{"full_name", "id", "username"}` per collaborator) that **excludes the
-post's own owner** — confirmed on both saved examples, including one where
-the owner (`dailyfashion_news`) is an external account with no ARTMS members
-in her own name, present only via `coauthors`. It is not in the metadata
-sidecar's `include` list today. Adding it would give collab detection a
-direct field instead of inferring it from filename/directory identity
-mismatches, and it never needs the "is this the owner" branch `coauthors`
-already excludes for us.
+**`coauthors` is a native, richer signal for this, and is now captured.**
+The raw API response carries a `coauthors` array (`{"full_name", "id",
+"username"}` per collaborator) that **excludes the post's own owner** —
+confirmed on both saved examples, including one where the owner
+(`dailyfashion_news`) is an external account with no ARTMS members in her
+own name, present only via `coauthors`. Added to the post-level metadata
+JSON's `include` list on 2026-09-01: a direct field instead of inferring a
+collab from filename/directory identity mismatches, and it never needs the
+"is this the owner" branch `coauthors` already excludes for us.
+
+**Per-carousel-item fields got their own sidecar, added the same day.**
+`width`, `height`, `width_original`, `height_original` and `tagged_users`
+live on the per-FILE kwdict, not the per-post one the metadata JSON above
+reads — a carousel's items can each have different dimensions and tags,
+which one post-level JSON can't represent. `gdl-sync.py`'s `media_pp` is a
+second `metadata` postprocessor, `event: "file"` (gallery-dl's default when
+omitted), so it runs once per downloaded file and writes `<filename>.json`
+alongside it — e.g. `... - 01.jpg.json` next to `... - 01.jpg`, never
+colliding with the post-level `....json`, which has no per-item number.
+`owner` — a full user object (profile pic URLs, privacy flags) for whoever
+posted that specific item — is deliberately left out, the same reasoning as
+`audio_user` above. Verified against the same two saved examples: correct
+per-item `width`/`height` and `tagged_users` came back on a live re-fetch of
+an already-archived carousel, with zero media re-downloaded (the existing
+skip-archive still applies; only the new sidecars are new files).
 
 ### The shared archive-db dedups media across profiles too, not just within one