feat: retire pixiv entirely — delete its code, its ledgers, its credential (3977, 3978, 3979)

Milestone #406 phase 2, with issue #3980 folded in. Phase 1 (2026-09-13)
unregistered pixiv so nothing could reach it; the code has sat in the tree
uncalled since. DeviantArt is why the second half is not left for later —
#3069 retired it in code on 2026-08-27 and its stored session was still in
the database seven weeks on.

Step 5 — the code. Deletes pixiv_client, pixiv_downloader, pixiv_ingester,
platforms/pixiv and their three test modules and fixture, then edits out
every remaining reference: the dispatch entry, the campaign-id and verify
branches in download_backends, the display-name branch in extension_service,
and the comments that still described pixiv as live.

The consolidation check the step asked for comes back negative:
native_ingest_common has seven non-pixiv callers (patreon, subscribestar,
membership_reconcile, membership_roster, ingest_core), so nothing there
drops to a single user.

Step 6 — the data, alembic 0102. Drops pixiv_seen_media and
pixiv_failed_media, and deletes credential rows whose platform is not
registered. Written as "not registered" rather than "pixiv" at the step's
explicit ask, which is what makes one migration cover two retirements: the
pixiv OAuth refresh token and DeviantArt's leftover session (#3980). It is
also the only way either row can go — the credentials UI renders one card
per platform from /api/platforms and looks the credential up by key, so an
unregistered platform's row has no card and no Remove button.

Pixiv's Source rows are KEPT, changing the milestone's original data table
on the operator's call. `platform` is stored only on Source; neither Post
nor ImageRecord carries it. Both FKs are ON DELETE SET NULL, so a delete
would not lose the art — but it would drop every pixiv image into the
gallery's __unsourced__ bucket and strip the platform chip off every pixiv
post. The rows stay disabled (0097) and unregistered, so nothing schedules
or downloads through them. Keeping them costs nothing and keeps the
attribution that "the art already downloaded from pixiv stays" is about.

Step 7 — the guard. test_pixiv_code_and_tables_are_gone asserts absence from
the module table and from Base.metadata, not from prose (snippet #3352's
trap). The extension and registry negative assertions were already in place
from phase 1. The final sweep found one real residue step 4 missed:
extension/README.md still advertised pixiv support and carried a "Pixiv
OAuth" manual-test item.

Also replaces the two deleted dispatch tests with one over the whole
NATIVE_INGESTER_PLATFORMS set, so adding a platform and forgetting its
ingester class now fails at unit level rather than as a mid-download
KeyError.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/gallery_service.py

@@ -45,7 +45,7 @@ from .tag_query import (
 # provenance (filesystem imports). Returned by facets() as a null-valued
 # bucket; the frontend maps that null back to this sentinel in the URL so the
 # bucket is selectable. Underscore-wrapped so it can't collide with a real
-# gallery-dl platform name (patreon/pixiv/...).
+# gallery-dl platform name (patreon/hentaifoundry/...).
 UNSOURCED_PLATFORM = "__unsourced__"