feat(artist): pixiv display-name at add-time + identity-by-source (#130 steps 2+3)

Final piece of the artist decoupling. (1) Identity-by-source: quick_add_source
resolves the artist by an existing (platform, url) Source first, so a re-add
reuses the artist even after it was renamed (its frozen slug no longer matches
the name) — a slug-based lookup would have duplicated it. (2) Pixiv naming: a new
pixiv source resolves the real display name via the app API (PixivClient
.resolve_display_name → /v1/user/detail) using the stored token, so the artist is
'Kurotsuchi Machi' not '12345678' — and its name-derived slug matches what a
native download produces, unifying them. Falls back to the numeric id when no
token/crypto. ExtensionService gains the crypto seam; the endpoint passes it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/extension.py

@@ -84,11 +84,15 @@ async def quick_add_source():
     if not isinstance(url, str) or not url.strip():
         return _bad("invalid_body", detail="url is required")
 
+    from .credentials import _get_crypto
+
     async with get_session() as session:
         if not await _ext_key_required(session):
             return _bad("unauthorized", status=401)
         try:
-            result = await ExtensionService(session).quick_add_source(url)
+            # crypto lets a pixiv add resolve the artist's display name via the
+            # stored OAuth token (else it falls back to the numeric id). #130.
+            result = await ExtensionService(session, _get_crypto()).quick_add_source(url)
         except UnknownPlatformError as exc:
             return _bad(
                 "unknown_platform",