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_api_extension.py

@@ -65,6 +65,47 @@ async def test_quick_add_source_idempotent(client, ext_key):
     assert body2["created_artist"] is False
 
 
+@pytest.mark.asyncio
+async def test_quick_add_reuses_source_artist_after_rename(client, ext_key):
+    # #130 identity-by-source: after renaming the artist (slug frozen ≠ new
+    # name-slug), re-adding the SAME source must reuse it — a slug-based lookup
+    # would miss and duplicate the artist.
+    url = "https://www.subscribestar.com/renamed-creator"
+    h = {"X-Extension-Key": ext_key}
+    a1 = (await (await client.post(
+        "/api/extension/quick-add-source", json={"url": url}, headers=h
+    )).get_json())["artist"]
+    await client.patch(f"/api/artists/{a1['id']}", json={"name": "Totally Different"})
+    b2 = await (await client.post(
+        "/api/extension/quick-add-source", json={"url": url}, headers=h
+    )).get_json()
+    assert b2["created_artist"] is False
+    assert b2["artist"]["id"] == a1["id"]
+    assert b2["artist"]["name"] == "Totally Different"
+
+
+@pytest.mark.asyncio
+async def test_resolve_artist_name_pixiv_uses_api(db, monkeypatch):
+    # #130: a pixiv add resolves the display name via the app API (with a token),
+    # not the numeric id; falls back to the id without crypto/token.
+    from backend.app.services.credential_service import CredentialService
+    from backend.app.services.extension_service import ExtensionService
+    from backend.app.services.pixiv_client import PixivClient
+
+    async def _tok(self, platform):
+        return "tok" if platform == "pixiv" else None
+    monkeypatch.setattr(CredentialService, "get_token", _tok)
+    monkeypatch.setattr(
+        PixivClient, "resolve_display_name", lambda self, uid: "Kurotsuchi Machi"
+    )
+
+    svc = ExtensionService(db, crypto=object())     # crypto unused (token stubbed)
+    assert await svc._resolve_artist_name("pixiv", "555") == "Kurotsuchi Machi"
+    assert await svc._resolve_artist_name("patreon", "alice") == "alice"   # passthrough
+    # No crypto → no resolution attempt → fall back to the raw handle.
+    assert await ExtensionService(db)._resolve_artist_name("pixiv", "555") == "555"
+
+
 @pytest.mark.parametrize("url,platform,slug", [
     ("https://www.patreon.com/maewix",                  "patreon",       "maewix"),
     ("https://patreon.com/maewix",                       "patreon",       "maewix"),

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():