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/gallery_dl.py

@@ -299,8 +299,9 @@ class GalleryDLService:
     # (services/patreon_ingester.py), not gallery-dl.
     PLATFORM_DEFAULTS = {
         # subscribestar removed — native-ingester platform now (#71); pixiv
-        # removed likewise (#129). The remaining entries are the gallery-dl
-        # platforms not yet migrated.
+        # 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.
         "hentaifoundry": {
             "content_types": ["all"],
             "directory": [],
@@ -316,15 +317,6 @@ class GalleryDLService:
             "reactions": False,
             "threads": True,
         },
-        "deviantart": {
-            "content_types": ["all"],
-            "directory": [],
-            "filename": "{index:>03}_{title[:50]}.{extension}",
-            "flat": True,
-            "original": True,
-            "mature": True,
-            "metadata": True,
-        },
     }
 
     def __init__(