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/services/pixiv_client.py

@@ -540,6 +540,19 @@ class PixivClient:
                 return
             current = str(next_url).rpartition("?")[2]
 
+    # -- user detail ---------------------------------------------------------
+
+    def resolve_display_name(self, user_id: str) -> str | None:
+        """The pixiv user's display name via `/v1/user/detail` (gallery-dl's
+        user_detail) — used to name the Artist when a source is added by numeric
+        id. None on any failure (the caller falls back to the id)."""
+        try:
+            body = self._call("/v1/user/detail", {"user_id": str(user_id)})
+        except PixivAPIError:
+            return None
+        name = (body.get("user") or {}).get("name") if isinstance(body, dict) else None
+        return name if isinstance(name, str) and name.strip() else None
+
     # -- verify --------------------------------------------------------------
 
     def verify_auth(self) -> tuple[bool | None, str]: