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>

tests/test_post_feed_service.py

@@ -169,7 +169,7 @@ async def test_scroll_filters_by_artist(db):
 async def test_scroll_filters_by_platform(db):
     artist = await _seed_artist(db, "alice-platf")
     src_p = await _seed_source(db, artist.id, "patreon", "https://p/alice-pp")
-    src_d = await _seed_source(db, artist.id, "deviantart", "https://d/alice-dd")
+    src_d = await _seed_source(db, artist.id, "discord", "https://d/alice-dd")
     now = datetime.now(UTC)
     pp = await _seed_post(db, src_p.id, external_id="PP", post_date=now)
     await _seed_post(db, src_d.id, external_id="PD", post_date=now)
@@ -234,7 +234,7 @@ async def test_scroll_combined_artist_and_platform(db):
     alice = await _seed_artist(db, "alice-combo")
     bob = await _seed_artist(db, "bob-combo")
     src_alice_patreon = await _seed_source(db, alice.id, "patreon", "https://p/alice-c")
-    src_alice_da = await _seed_source(db, alice.id, "deviantart", "https://d/alice-c")
+    src_alice_da = await _seed_source(db, alice.id, "discord", "https://d/alice-c")
     src_bob_patreon = await _seed_source(db, bob.id, "patreon", "https://p/bob-c")
     now = datetime.now(UTC)
     target = await _seed_post(