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

tests/test_pixiv_client.py

@@ -390,6 +390,24 @@ def test_verify_auth_bad_token():
     assert "rotate" in message.lower() or "rejected" in message.lower()
 
 
+def test_resolve_display_name(client, monkeypatch):
+    monkeypatch.setattr(
+        client, "_call",
+        lambda e, p: {"user": {"name": "Kurotsuchi Machi", "id": p["user_id"]}},
+    )
+    assert client.resolve_display_name("99") == "Kurotsuchi Machi"
+
+
+def test_resolve_display_name_none_on_failure(client, monkeypatch):
+    def boom(endpoint, params):
+        raise PixivAPIError("nope", status_code=404)
+    monkeypatch.setattr(client, "_call", boom)
+    assert client.resolve_display_name("99") is None
+    # Empty/whitespace name → None (caller falls back to the id).
+    monkeypatch.setattr(client, "_call", lambda e, p: {"user": {"name": "  "}})
+    assert client.resolve_display_name("99") is None
+
+
 # -- rating ------------------------------------------------------------------------
 
 def test_rating_label():