refactor(platforms): retire deviantart end-to-end (#3069)

Executes the 2026-07-05 product decision (FC downloaders = art-dedicated
services only), which removed Twitter/X and Bluesky but left deviantart
fully wired for seven weeks — the half-retired state rule 22 exists to
prevent.

Removed: the PlatformInfo module and its registry entry, the gallery-dl
extractor block, extension_service's artist-page pattern, the extension's
PLATFORMS + PLATFORM_ARTIST_PATTERNS entries, its manifest host permission
and content-script match, the frontend icon/colour/label, and the operator-
facing "supported platforms" list that still advertised it.

Two judgment calls, both recorded in migration 0088:

  * existing `source` rows are DISABLED, not deleted. The row is the only
    record of the artist's DeviantArt URL. Disabling is also required for
    correctness rather than tidiness: with the platform unregistered the
    download path falls through to gallery-dl, which carries its OWN
    deviantart extractor, so an enabled row would have kept downloading
    from a dropped platform.
  * the `credential` row IS deleted — a live session cookie for a site FC
    will never call again.

Adds the invariant whose absence is why manifest.json drifted in the first
place: nothing tied its domain lists back to the platform table. The
extension suite now asserts both directions, plus that no host permission
belongs to an unclaimed domain (`*://*/*` exempted — FC is self-hosted at
an operator-chosen URL the extension cannot enumerate).

Extension version 1.0.10 -> 1.0.11: ci.yml's guard hard-fails a packaged
extension change without a bump. No release is cut — build.yml's
sign-extension job only runs on main.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/services/platforms/__init__.py

@@ -8,9 +8,10 @@ PLATFORMS below. Sidecar parsing, cookie materialization, and
 
 Lifted from GallerySubscriber's
 ~/Nextcloud/Projects/GallerySubscriber/backend/app/api/platforms.py
-and ~/.../extension/lib/platforms.js. Six platforms; auth_type and
+and ~/.../extension/lib/platforms.js. Five platforms; auth_type and
 URL patterns match GS exactly so the existing browser extension
-hits FC unmodified.
+hits FC unmodified. deviantart was dropped at #3069 (2026-08-27) —
+FC downloaders are art-dedicated services only.
 """
 
 from .base import (
@@ -18,7 +19,6 @@ from .base import (
     DEFAULT_EXTERNAL_POST_ID_KEYS,
     PlatformInfo,
 )
-from .deviantart import INFO as _DEVIANTART
 from .discord import INFO as _DISCORD
 from .hentaifoundry import INFO as _HENTAIFOUNDRY
 from .patreon import INFO as _PATREON
@@ -33,7 +33,6 @@ PLATFORMS: dict[str, PlatformInfo] = {
         _HENTAIFOUNDRY,
         _DISCORD,
         _PIXIV,
-        _DEVIANTART,
     )
 }