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>

tests/test_post_feed_service.py

@@ -283,6 +283,27 @@ async def test_scroll_item_shape_minimal(db):
     assert "description_full" not in item
 
 
+@pytest.mark.asyncio
+async def test_a_discord_post_names_its_channel(db):
+    """#4481: the card says which channel a message came from. Read from the
+    record's `channel`; never on another platform, even with the same key."""
+    artist = await _seed_artist(db, "todding-ch")
+    dsrc = await _seed_source(db, artist.id, "discord", "https://discord.com/channels/1/2")
+    psrc = await _seed_source(db, artist.id, "patreon", "https://p/todding-ch")
+    now = datetime.now(UTC)
+    d = await _seed_post(db, dsrc.id, external_id="D1", post_date=now)
+    d.raw_metadata = {"category": "discord", "channel": "banana-land"}
+    blank = await _seed_post(db, dsrc.id, external_id="D2", post_date=now)
+    blank.raw_metadata = {"category": "discord", "channel": "  "}
+    p = await _seed_post(db, psrc.id, external_id="P1", post_date=now)
+    p.raw_metadata = {"channel": "not-discord"}
+    await db.commit()
+
+    page = await PostFeedService(db).scroll(cursor=None, limit=10, artist_id=artist.id)
+    by_id = {it["external_post_id"]: it["channel"] for it in page["items"]}
+    assert by_id == {"D1": "banana-land", "D2": None, "P1": None}
+
+
 @pytest.mark.asyncio
 async def test_scroll_surfaces_translation_fields(db):
     # #143: a translated post exposes the translated title/description + source