fix: a Discord source takes only messages with an image attached, from the posters it names, and knows its channel's name (#4481)

- A message with no image or video attachment is chat: extract_media returns
  nothing for it, so nothing downloads and no post record is written. A
  message that passes keeps every file, numbered as gallery-dl numbers them.
- `discord_authors` in a source's config limits the walk to those posters
  (id, username or display name); the edit dialog has a field for it and no
  longer drops config keys it has no field for.
- source.display_name (migration 0115), refreshed by every Discord walk, shows
  on Subscriptions in place of the two-id URL; a Discord post card names its
  channel from the record's `channel`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/post_feed_service.py

@@ -479,6 +479,10 @@ class PostFeedService:
             # the UI uses this to explain why a post it linked to is not in the
             # stream.
             "absorbed_by_post_id": post.absorbed_by_post_id,
+            # #4481. The Discord channel a message was posted in: both the native
+            # post record and gallery-dl's sidecars carry it as `channel`. None
+            # on every other platform, and on a Discord post that never said.
+            "channel": _discord_channel(post, source),
             "artist": {"id": artist.id, "name": artist.name, "slug": artist.slug},
             "source": (
                 {"id": source.id, "platform": source.platform}
@@ -488,3 +492,13 @@ class PostFeedService:
             "thumbnails_more": thumbs_entry["more"],
             "attachments": atts_map.get(post.id, []),
         }
+
+
+def _discord_channel(post: Post, source: Source | None) -> str | None:
+    if source is None or source.platform != "discord":
+        return None
+    raw = post.raw_metadata if isinstance(post.raw_metadata, dict) else {}
+    channel = raw.get("channel")
+    if not isinstance(channel, str):
+        return None
+    return channel.strip() or None