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_api_extension.py

@@ -129,7 +129,6 @@ async def test_resolve_artist_name_dispatches_per_platform(db, monkeypatch):
     ("https://www.subscribestar.com/foobar",             "subscribestar", "foobar"),
     ("https://subscribestar.adult/foobar",               "subscribestar", "foobar"),
     ("https://www.hentai-foundry.com/user/Foo/profile",  "hentaifoundry", "Foo"),
-    ("https://www.deviantart.com/baz",                   "deviantart",    "baz"),
     ("https://www.pixiv.net/users/12345",                "pixiv",         "12345"),
     ("https://www.pixiv.net/en/users/12345",             "pixiv",         "12345"),
 ])
@@ -160,6 +159,23 @@ async def test_quick_add_source_unknown_url_400(client, ext_key):
     assert "known" in body
 
 
+@pytest.mark.asyncio
+async def test_quick_add_source_rejects_retired_deviantart(client, ext_key):
+    """#3069: a DeviantArt creator URL used to derive cleanly. Now that the
+    platform is retired, the extension's own gate should never offer the
+    button — but a stale content script on an un-updated browser still can,
+    so the backend has to refuse it rather than create an unusable source."""
+    resp = await client.post(
+        "/api/extension/quick-add-source",
+        json={"url": "https://www.deviantart.com/baz"},
+        headers={"X-Extension-Key": ext_key},
+    )
+    assert resp.status_code == 400
+    body = await resp.get_json()
+    assert body["error"] == "unknown_platform"
+    assert "deviantart" not in body["known"]
+
+
 @pytest.mark.asyncio
 async def test_quick_add_source_invalid_url_400(client, ext_key):
     resp = await client.post(