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/test_gdl_sync.py

@@ -232,6 +232,37 @@ class PostUrl(unittest.TestCase):
             self.assertNotIn(str(staging / subcategory), cmd)
 
 
+class MetadataFields(unittest.TestCase):
+    """Extra fields captured from the raw API response, verified against two
+    saved real examples on 2026-09-01 (see docs/gallery-dl.md)."""
+
+    def test_post_level_json_captures_coauthors(self):
+        config = gdl.build_config("1M", [6.0, 10.0], [3.0, 6.0])
+        pps = config["extractor"]["instagram"]["posts"]["postprocessors"]
+        post_json = next(pp for pp in pps if pp.get("filename", "").endswith(".json"))
+        self.assertIn("coauthors", post_json["include"])
+
+    def test_per_item_dimensions_and_tags_get_their_own_sidecar(self):
+        config = gdl.build_config("1M", [6.0, 10.0], [3.0, 6.0])
+        pps = config["extractor"]["instagram"]["posts"]["postprocessors"]
+        # No "filename" of its own -- unlike the other two, which are keyed
+        # by POST_STEM -- because it must vary per carousel item, not per post.
+        media_pp = next(pp for pp in pps if "filename" not in pp)
+        for field in ("width", "height", "width_original", "height_original",
+                      "tagged_users", "shortcode", "num"):
+            self.assertIn(field, media_pp["include"])
+        # `owner` is a full user object (profile pic URLs, privacy flags) for
+        # whoever posted that item -- deliberately excluded, same reasoning
+        # as `audio_user` on the post-level json.
+        self.assertNotIn("owner", media_pp["include"])
+
+    def test_reels_get_the_same_capture_as_posts(self):
+        config = gdl.build_config("1M", [6.0, 10.0], [3.0, 6.0])
+        posts_pps = config["extractor"]["instagram"]["posts"]["postprocessors"]
+        reels_pps = config["extractor"]["instagram"]["reels"]["postprocessors"]
+        self.assertEqual(posts_pps, reels_pps)
+
+
 class UrlsFile(unittest.TestCase):
     def test_reads_every_form_a_person_might_paste(self):
         with tempfile.TemporaryDirectory() as d: