feat: the extension picks a Discord source's posters from who posts in the channel, stored by id (#4488)

On a Discord channel page the Add panel lists who posted in the newest 200
messages, the creator the server is named for (or owned by) ticked. On a
channel FabledCurator already follows, the chip opens the same list with Save
and Open artist. The list is kept as user ids, so a rename never quietly stops
it matching; the name each was picked under rides beside it for display, and
the Subscriptions dialog shows it.

- DiscordClient.recent_posters + rank_posters (owner / name-matches-server;
  bots never suggested; it suggests, the operator ticks).
- GET/POST /api/extension/discord/posters; quick-add takes discord_authors.

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

backend/app/api/extension.py

@@ -19,8 +19,10 @@ from ..models import AppSetting
 from ..services.extension_service import (
     ExtensionService,
     InvalidUrlError,
+    PosterLookupError,
     UnknownArtistError,
     UnknownPlatformError,
+    UnknownSourceError,
 )
 from ..services.source_service import KNOWN_PLATFORMS
 from ._responses import error_response as _bad
@@ -121,6 +123,10 @@ async def quick_add_source():
     # Patreon is canon: adding a Patreon source to an existing artist can take
     # the creator's Patreon display name (name only; the slug never moves).
     use_platform_name = body.get("use_platform_name") is True
+    # #4488: a Discord add can carry the posters picked in the panel.
+    discord_authors = body.get("discord_authors")
+    if discord_authors is not None and not _valid_authors(discord_authors):
+        return _bad("invalid_body", detail="discord_authors must be a list of {id, name}")
 
     from .credentials import _get_crypto
 
@@ -133,6 +139,7 @@ async def quick_add_source():
             result = await ExtensionService(session, _get_crypto()).quick_add_source(
                 url, artist_id=artist_id, artist_name=artist_name,
                 use_platform_name=use_platform_name,
+                discord_authors=discord_authors,
             )
         except UnknownArtistError as exc:
             return _bad("not_found", detail=str(exc), status=404)
@@ -147,6 +154,59 @@ async def quick_add_source():
     return jsonify(result), (201 if result["created_source"] else 200)
 
 
+def _valid_authors(value) -> bool:
+    """[{id, name}]: the id is what the list keys on, the name only shows it."""
+    return isinstance(value, list) and all(
+        isinstance(a, dict) and isinstance(a.get("id"), str | int)
+        and not isinstance(a.get("id"), bool)
+        and (a.get("name") is None or isinstance(a.get("name"), str))
+        for a in value
+    )
+
+
+@extension_bp.route("/discord/posters", methods=["GET"])
+async def discord_posters():
+    """Who posts in the Discord channel `url` shows, lately, with the likely
+    creator marked (#4488). Reads the channel with the stored token; writes
+    nothing."""
+    url = (request.args.get("url") or "").strip()
+    if not url:
+        return _bad("invalid_body", detail="url query parameter is required")
+    from .credentials import _get_crypto
+
+    async with get_session() as session:
+        if not await _ext_key_required(session):
+            return _bad("unauthorized", status=401)
+        try:
+            result = await ExtensionService(session, _get_crypto()).discord_posters(url)
+        except (InvalidUrlError, UnknownPlatformError) as exc:
+            return _bad("invalid_url", detail=str(exc))
+        except PosterLookupError as exc:
+            return _bad("posters_unavailable", detail=str(exc), status=409)
+    return jsonify(result)
+
+
+@extension_bp.route("/discord/posters", methods=["POST"])
+async def set_discord_posters():
+    """Set the poster list of the source following `url`'s channel."""
+    body = await request.get_json(silent=True)
+    if not isinstance(body, dict) or not isinstance(body.get("url"), str):
+        return _bad("invalid_body", detail="url is required")
+    authors = body.get("authors")
+    if not _valid_authors(authors):
+        return _bad("invalid_body", detail="authors must be a list of {id, name}")
+    async with get_session() as session:
+        if not await _ext_key_required(session):
+            return _bad("unauthorized", status=401)
+        try:
+            result = await ExtensionService(session).set_discord_posters(body["url"], authors)
+        except (InvalidUrlError, UnknownPlatformError, PosterLookupError) as exc:
+            return _bad("invalid_url", detail=str(exc))
+        except UnknownSourceError as exc:
+            return _bad("not_found", detail=str(exc), status=404)
+    return jsonify(result)
+
+
 def _read_manifest_sync() -> dict | None:
     """All the filesystem-touching work for /api/extension/manifest,
     in a sync helper so the async route can dispatch it via