feat: finish the Discord switchover — recapture on every native source, backfills that run, gallery-dl's Discord config retired (milestone 428)

- Recover and Recapture show on every native source. The menu gated them on
  a copied platform list ('patreon', 'subscribestar') that went stale when
  Discord moved over. Sources now carry `native_ingester` from the backend's
  own predicate.
- A running backfill is due on every scheduler tick. Nothing queued a
  backfill's next chunk: each one waited for the source's regular interval,
  so an armed backfill sat idle until the next check (8h at the default) and
  a five-chunk walk took most of two days. The in-flight guard and the
  platform lock keep one chunk at a time. A failing source falls back to its
  backoff, and a stalled or out-of-budget walk stops being due. It also runs
  when the artist has auto-check off, since the operator started it by hand.
- gallery-dl no longer carries Discord: its naming constants, platform
  defaults, sidecar-mirroring postprocessor and token injection are gone.
  The naming test moves to the native downloader and still renders against
  the real gallery-dl sidecar fixture. That is the guard that the files
  gallery-dl wrote are found on disk rather than fetched again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/gallery_dl.py

@@ -95,48 +95,6 @@ BACKFILL_CHUNK_SECONDS = 600
 _DEFAULT_GDL_TIMEOUT_SECONDS = 870
 
 
-# --- Discord naming ---------------------------------------------------------
-#
-# Derived from a REAL sidecar (operator's instance, 2026-09-13), not from memory
-# of gallery-dl's extractor. What gallery-dl's discord extractor actually emits
-# for an attachment: `channel` is a plain STRING (the channel's name), the
-# message is `message_id`, the attachment's position in it is `num`, and there
-# is NO `id` key at all.
-#
-# The previous patterns asked for `{channel[name]}` and `{id}`. Both render as
-# "None", so every Discord download since the platform was added landed in a
-# directory called `None` as `<date>_None_<original name>`. Worse, the sidecar was
-# named `{filename}.json` — the attachment's ORIGINAL name — which (a) `find_
-# sidecar` can never pair with `<date>_None_<name>.png`, so no Discord file ever
-# got a Post or a post date, and (b) collides: every `image.png` in a channel
-# overwrote the same `image.json`, so the one sidecar that survived described
-# whichever message happened to be written last.
-#
-# The fix names the sidecar EXACTLY like the media minus its extension, so
-# `find_sidecar`'s first candidate (`media.with_suffix(".json")`) is the match
-# and the name is unique per attachment. tests/test_gallery_dl_naming.py renders
-# these patterns against a sanitized copy of the real sidecar, so a key that
-# does not exist fails CI instead of silently becoming "None".
-DISCORD_FILENAME = "{date:%Y%m%d}_{message_id}_{num:>02}_{filename}.{extension}"
-DISCORD_DIRECTORY = ["{channel}"]
-
-
-def sidecar_name_for(media_pattern: str) -> str | None:
-    """The metadata filename pattern that names a sidecar exactly like its media.
-
-    Returns None for a pattern that does not end in `.{extension}`, since then
-    there is no media stem to mirror and the caller must fall back.
-    """
-    suffix = ".{extension}"
-    if not media_pattern.endswith(suffix):
-        return None
-    return media_pattern[: -len(suffix)] + ".json"
-
-
-def metadata_postprocessor(filename: str) -> dict:
-    return {"name": "metadata", "mode": "json", "directory": ".", "filename": filename}
-
-
 def archive_path(images_root: Path) -> Path:
     """gallery-dl's download archive: the record of what it has already fetched.
 
@@ -425,27 +383,16 @@ class GalleryDLService:
     # (services/patreon_ingester.py), not gallery-dl.
     PLATFORM_DEFAULTS = {
         # subscribestar removed — native-ingester platform now (#71); pixiv
-        # removed likewise (#129); deviantart removed at #3069 as a dropped
-        # platform, not a migrated one. The remaining entries are the
-        # gallery-dl platforms not yet migrated.
+        # removed likewise (#129); discord likewise (milestone 428, whose
+        # downloader keeps this config's on-disk naming); deviantart removed at
+        # #3069 as a dropped platform, not a migrated one. HentaiFoundry is the
+        # one platform left here, by the operator's choice not to migrate it.
         "hentaifoundry": {
             "content_types": ["all"],
             "directory": [],
             "filename": "{category}_{index:>03}_{title[:50]}.{extension}",
             "include": "all",
         },
-        "discord": {
-            "content_types": ["all"],
-            "directory": DISCORD_DIRECTORY,
-            "filename": DISCORD_FILENAME,
-            # Overrides the global `{filename}.json` sidecar for this extractor
-            # only — see the Discord naming note above.
-            "postprocessors": [metadata_postprocessor(sidecar_name_for(DISCORD_FILENAME))],
-            "embeds": "all",
-            "stickers": True,
-            "reactions": False,
-            "threads": True,
-        },
     }
 
     def __init__(
@@ -560,17 +507,6 @@ class GalleryDLService:
         if source_config.filename_pattern:
             platform_section["filename"] = source_config.filename_pattern
 
-        # A platform that names its sidecar after its media must keep doing so
-        # under a per-source filename override, or the pairing breaks exactly the
-        # way Discord's did. No metadata wanted means no platform postprocessor
-        # either — the global list was already dropped above.
-        if "postprocessors" in platform_section:
-            mirrored = sidecar_name_for(platform_section.get("filename") or "")
-            if not source_config.save_metadata or mirrored is None:
-                platform_section.pop("postprocessors")
-            else:
-                platform_section["postprocessors"] = [metadata_postprocessor(mirrored)]
-
         platform_section["metadata"] = source_config.save_metadata
 
         return config
@@ -818,9 +754,6 @@ class GalleryDLService:
 
         if cookies_path:
             config["extractor"]["cookies"] = cookies_path
-        if auth_token and platform == "discord":
-            config["extractor"].setdefault("discord", {})
-            config["extractor"]["discord"]["token"] = auth_token
 
         with tempfile.NamedTemporaryFile(
             mode="w", suffix=".json", delete=False, dir=str(self._config_dir),
@@ -1004,8 +937,6 @@ class GalleryDLService:
         config = self._build_config_for_source(platform, source_config, artist_slug)
         if cookies_path:
             config["extractor"]["cookies"] = cookies_path
-        if auth_token and platform == "discord":
-            config["extractor"].setdefault("discord", {})["token"] = auth_token
 
         with tempfile.NamedTemporaryFile(
             mode="w", suffix=".json", delete=False, dir=str(self._config_dir),