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

scripts/gdl-sync.py

@@ -325,6 +325,32 @@ def build_config(rate: str, sleep_request: list[float],
             "post_shortcode", "post_id", "type", "date", "post_date",
             "username", "fullname", "owner_id", "description", "count",
             "likes", "post_url", "sidecar_shortcode",
+            # A Collab's `username`/`fullname`/`owner_id` above are always the
+            # original poster's, never the scraped account's -- see
+            # docs/gallery-dl.md. `coauthors` is the API's own list of every
+            # OTHER collaborator (it excludes the post's owner), a direct
+            # signal instead of inferring a collab from identity mismatches.
+            "coauthors",
+        ],
+    }
+    # Per-carousel-item fields (`width`/`height`/`tagged_users`/`owner`) live
+    # on the per-FILE kwdict, not the per-post one `meta_pp` reads -- a
+    # carousel's items can each have different dimensions and tags, which one
+    # post-level JSON can't represent. `owner` is deliberately left out: it is
+    # a full user object (profile pic URLs, privacy flags) for whoever posted
+    # that specific item, the same reason `audio_user` is excluded above.
+    media_pp = {
+        "name": "metadata",
+        "mode": "json",
+        # event defaults to "file" when omitted -- runs once per downloaded
+        # file, appending ".json" to that file's own full name, e.g.
+        # "... - 01.jpg" gets "... - 01.jpg.json" alongside it. Never
+        # collides with the post-level "....json" above, which has no
+        # per-item number.
+        "include": [
+            "media_id", "shortcode", "num",
+            "width", "height", "width_original", "height_original",
+            "tagged_users",
         ],
     }
 
@@ -363,6 +389,7 @@ def build_config(rate: str, sleep_request: list[float],
             "postprocessors": [
                 {**caption_pp, "filename": stem + ".txt"},
                 {**meta_pp, "filename": stem + ".json"},
+                media_pp,
             ],
         }