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/models/source.py

@@ -46,6 +46,11 @@ class Source(Base):
     enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="true")
 
     config_overrides: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    # alembic 0115: what the source is called on its platform, where the URL
+    # alone doesn't say — a Discord link is two numbers. The ingester that
+    # walks it refreshes it every walk, so a rename reads as the new name.
+    # NULL until a walk has read it, and on platforms that don't write it.
+    display_name: Mapped[str | None] = mapped_column(Text, nullable=True)
 
     last_checked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     last_error: Mapped[str | None] = mapped_column(Text, nullable=True)