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/discord_ingester.py

@@ -13,6 +13,11 @@ Two things differ from the cookie platforms:
     something; a Discord drop is routinely files and nothing else, so on
     Discord that is an ordinary backfill, not a broken parser.
 
+And two things only a Discord source has: `discord_authors` in its
+config_overrides limits it to the creator's own messages (a server's other
+members post pictures too), and it learns its own name — the server and
+channel it walks — into `source.display_name`.
+
 `campaign_id` is the source URL (a server, channel, thread or category link).
 FC runs on a plain-HTTP homelab; nothing here uses a secure-context Web API.
 """
@@ -20,15 +25,23 @@ FC runs on a plain-HTTP homelab; nothing here uses a secure-context Web API.
 from __future__ import annotations
 
 import asyncio
+import logging
 from collections.abc import Callable
 from pathlib import Path
 
-from ..models import DiscordFailedMedia, DiscordSeenMedia
+from sqlalchemy import select, update
+
+from ..models import DiscordFailedMedia, DiscordSeenMedia, Source
 from .discord_client import DiscordAPIError, DiscordClient, MediaItem
 from .discord_downloader import DiscordDownloader
 from .ingest_core import Ingester
 
+log = logging.getLogger(__name__)
+
 _LEDGER_KEY_MAX = 128
+# `config_overrides` key: the people whose messages this source takes (user
+# ids, usernames or display names). Absent or empty takes everyone's.
+AUTHORS_KEY = "discord_authors"
 
 
 def _ledger_key(media: MediaItem) -> str:
@@ -74,6 +87,46 @@ class DiscordIngester(Ingester):
             body_canary=False,
         )
 
+    def run(self, **kwargs):
+        """The core walk, bracketed by the two things only a Discord source
+        has: whose messages it takes, read before the walk, and the name of
+        what it walks, written after it (the walk is what loads that name)."""
+        source_id = kwargs["source_id"]
+        self.client.only_from(self._source_authors(source_id))
+        try:
+            return super().run(**kwargs)
+        finally:
+            self._record_display_name(source_id, kwargs["campaign_id"])
+
+    def _source_authors(self, source_id: int) -> list:
+        if self.session_factory is None:
+            return []
+        with self.session_factory() as session:
+            overrides = session.execute(
+                select(Source.config_overrides).where(Source.id == source_id)
+            ).scalar_one_or_none() or {}
+        authors = overrides.get(AUTHORS_KEY) or []
+        return authors if isinstance(authors, list) else []
+
+    def _record_display_name(self, source_id: int, url: str) -> None:
+        """Refreshed on every walk, never written once: a renamed channel
+        should read as its new name. A name the walk couldn't read leaves the
+        stored one alone, and a failure here never fails the walk."""
+        label = self.client.source_label(url)
+        if not label or self.session_factory is None:
+            return
+        try:
+            with self.session_factory() as session:
+                session.execute(
+                    update(Source)
+                    .where(Source.id == source_id)
+                    .where(Source.display_name.is_distinct_from(label))
+                    .values(display_name=label)
+                )
+                session.commit()
+        except Exception as exc:  # a name is decoration — never fail the walk
+            log.warning("Discord: couldn't record source %s's name: %s", source_id, exc)
+
 
 async def verify_discord_credential(url: str, auth_token: str | None) -> tuple[bool | None, str]:
     """The uniform `(ok, message)` probe: is the token valid, and can its