fix(metadata): get series from api

tests/test_import_flow.py

@@ -225,3 +225,42 @@ def test_scan_rel_dir_and_request_update(client):
     assert updated["narrator"] == "Rufus Beck"
     assert updated["series"] == "HP"
     assert client.get("/api/requests", params={"status": "missing"}).json()[0]["narrator"] == "Rufus Beck"
+
+
+def test_retag_after_update(client):
+    from mutagen.id3 import ID3
+
+    root = client.tmp_path / "library" / "retag"
+    lib_id = client.post("/api/libraries", json={
+        "name": "Retag", "media_type": "audiobook", "root_path": str(root),
+    }).json()["id"]
+    req_id = client.post("/api/requests", json={
+        "library_id": lib_id, "title": "Der Ickabog", "authors": "J.K. Rowling",
+    }).json()["id"]
+
+    book_dir = client.downloads / "Der Ickabog"
+    book_dir.mkdir()
+    for i in range(2):
+        (book_dir / f"t{i}.mp3").write_bytes(b"")
+    items = client.get("/api/import/scan").json()["items"]
+    client.post("/api/import", json={"items": [{
+        "path": items[0]["path"], "is_dir": True,
+        "files": items[0]["files"], "request_id": req_id,
+    }]})
+
+    # no series yet -> tag absent
+    dest = root / "J.K. Rowling" / "Der Ickabog"
+    first = sorted(dest.iterdir())[0]
+    assert "TXXX:SERIES" not in ID3(first)
+
+    # add series + narrator, then retag
+    client.put(f"/api/requests/{req_id}", json={
+        "library_id": lib_id, "title": "Der Ickabog", "authors": "J.K. Rowling",
+        "narrator": "Ben Becker", "series": "Rowling Kinderbücher", "volume": 2,
+    })
+    res = client.post(f"/api/requests/{req_id}/retag").json()
+    assert res == {"ok": True, "files": 2}
+    tags = ID3(first)
+    assert str(tags["TXXX:SERIES"]) == "Rowling Kinderbücher"
+    assert str(tags["TXXX:SERIES-PART"]) == "2"
+    assert str(tags["TCOM"]) == "Ben Becker"