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>

frontend/src/components/settings/BrowserExtensionCard.vue

@@ -9,7 +9,7 @@
     <v-card-text>
       <p class="fc-muted text-body-2">
         Pushes session cookies from supported platforms
-        (patreon, subscribestar, hentaifoundry, discord, pixiv, deviantart)
+        (patreon, subscribestar, hentaifoundry, discord, pixiv)
         into FabledCurator, and lets you add a creator as a source from
         their page in one click.
       </p>

frontend/src/utils/platformColor.js

@@ -1,8 +1,10 @@
 // Single source of truth for platform → color + icon mapping. Used by
-// PlatformChip and any other GS-style platform-tagged surface. The six
+// PlatformChip and any other GS-style platform-tagged surface. The five
 // platforms FC supports map 1:1 to the GS palette; unknown platforms fall
-// back to grey + mdi-web. Operator-confirmed scope 2026-05-27. The ICONS key
-// set is pinned against backend known_platform_keys() by
+// back to grey + mdi-web — which is deliberately what a retired platform
+// hits: a pre-#3069 deviantart source row still renders, as its raw key on
+// a grey chip. Operator-confirmed scope 2026-05-27. The ICONS key set is
+// pinned against backend known_platform_keys() by
 // tests/test_fe_be_contract.py.
 
 const ICONS = {
@@ -11,7 +13,6 @@ const ICONS = {
   hentaifoundry: 'mdi-palette',
   discord:       'mdi-discord',
   pixiv:         'mdi-alpha-p-box',
-  deviantart:    'mdi-deviantart',
 }
 
 const COLORS = {
@@ -20,7 +21,6 @@ const COLORS = {
   hentaifoundry: 'purple',
   discord:       'indigo',
   pixiv:         'blue',
-  deviantart:    'green',
 }
 
 const LABELS = {
@@ -29,7 +29,6 @@ const LABELS = {
   hentaifoundry: 'HentaiFoundry',
   discord:       'Discord',
   pixiv:         'Pixiv',
-  deviantart:    'DeviantArt',
 }
 
 export function platformIcon(platform) {