fix: a retired platform leaves the chip maps, and a fixture stops creating pixiv sources (406)

e3fd8c6 failed two lanes, both on things pixiv's retirement correctly started refusing.

Unit - test_fe_be_contract pins platformColor.js's ICONS keys to known_platform_keys(). I had kept pixiv's icon, colour and label "so existing pixiv posts don't look broken". That was wrong, and the file's own header already said why: unknown platforms fall back to a grey chip with the raw key, and that fallback is deliberately what a retired platform hits - it is how a pre-3069 deviantart row already renders. So pixiv leaves all three maps, and its posts show as a grey "pixiv" chip rather than a broken one. The header now says outright that a retired platform is removed, never kept, since the milestone plan itself got this backwards.

Integration - four reassign tests built their fixture source through SourceService.create with platform="pixiv", which the validator now rejects. Reassign never reads the platform and never moves files, so any registered platform serves; the fixture uses hentaifoundry. 1294 other integration tests passed on e3fd8c6, so nothing else used pixiv through a validating path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

tests/test_source_service.py

@@ -388,14 +388,17 @@ async def test_update_while_enabled_keeps_failure_state(db):
 async def _source_with_content(db, svc, artist):
     """A source under `artist` with one post + one image it contributed."""
     from backend.app.models import ImageProvenance, ImageRecord, Post
+    # Any registered platform will do — reassign never reads the platform, and
+    # never moves files (the storage path is immutable). This used pixiv until
+    # pixiv was retired (milestone #406) and `create` began refusing it.
     rec = await svc.create(
-        artist_id=artist.id, platform="pixiv",
-        url=f"https://www.pixiv.net/users/{artist.id}",
+        artist_id=artist.id, platform="hentaifoundry",
+        url=f"https://www.hentai-foundry.com/user/{artist.slug}/profile",
     )
     post = Post(source_id=rec.id, artist_id=artist.id, external_post_id="p1")
     db.add(post)
     img = ImageRecord(
-        path=f"/images/{artist.slug}/pixiv/pixiv/1_a_00.jpg",
+        path=f"/images/{artist.slug}/hentaifoundry/hentaifoundry/1_a_00.jpg",
         sha256=str(artist.id).rjust(64, "0"), size_bytes=1, mime="image/jpeg",
         width=1, height=1, origin="imported_filesystem",
         integrity_status="unknown", artist_id=artist.id,