fix: handle SoundCloud API 5xx errors with client_id refresh, backoff, and cursor fallback

SoundCloud began rejecting the fabricated pagination cursor with 500
errors. Fixed cursor user_id padding (zfill 22→20) to match the
documented format, added 5xx retry with exponential backoff in _api_get,
and added a fallback in fetch_likes that drops the fabricated cursor
when it causes persistent 500s.

Made-with: Cursor

tests/test_soundcloud.py

@@ -1,6 +1,8 @@
 import re
 from datetime import datetime, timezone
+from unittest.mock import patch
 
+import httpx
 import pytest
 
 from ntr_fetcher.soundcloud import SoundCloudClient
@@ -151,3 +153,76 @@ async def test_fetch_likes_retries_on_401(httpx_mock):
         until=datetime(2026, 3, 10, 0, 0, 0, tzinfo=timezone.utc),
     )
     assert len(tracks) == 1
+
+
+@pytest.mark.asyncio
+@patch("ntr_fetcher.soundcloud.asyncio.sleep", return_value=None)
+async def test_fetch_likes_retries_on_500(mock_sleep, httpx_mock):
+    httpx_mock.add_response(url="https://soundcloud.com", text=FAKE_HTML)
+    httpx_mock.add_response(
+        url=re.compile(r"https://api-v2\.soundcloud\.com/users/206979918/likes.*"),
+        status_code=500,
+    )
+    httpx_mock.add_response(
+        url="https://soundcloud.com",
+        text=FAKE_HTML.replace("test_client_id_abc123", "fresh_client_id_789"),
+    )
+    httpx_mock.add_response(
+        url=re.compile(r"https://api-v2\.soundcloud\.com/users/206979918/likes.*"),
+        json=FAKE_LIKES_RESPONSE,
+    )
+    client = SoundCloudClient()
+    tracks = await client.fetch_likes(
+        user_id=206979918,
+        since=datetime(2026, 3, 1, 0, 0, 0, tzinfo=timezone.utc),
+        until=datetime(2026, 3, 10, 0, 0, 0, tzinfo=timezone.utc),
+    )
+    assert len(tracks) == 1
+    mock_sleep.assert_called_once_with(1)
+
+
+@pytest.mark.asyncio
+@patch("ntr_fetcher.soundcloud.asyncio.sleep", return_value=None)
+async def test_fetch_likes_falls_back_to_no_cursor_on_persistent_500(mock_sleep, httpx_mock):
+    """When the fabricated cursor causes persistent 500s, fall back to no cursor."""
+    httpx_mock.add_response(url="https://soundcloud.com", text=FAKE_HTML)
+    for _ in range(3):
+        httpx_mock.add_response(
+            url=re.compile(r"https://api-v2\.soundcloud\.com/users/206979918/likes.*"),
+            status_code=500,
+        )
+        httpx_mock.add_response(url="https://soundcloud.com", text=FAKE_HTML)
+    httpx_mock.add_response(
+        url=re.compile(r"https://api-v2\.soundcloud\.com/users/206979918/likes.*"),
+        json=FAKE_LIKES_RESPONSE,
+    )
+    client = SoundCloudClient()
+    tracks = await client.fetch_likes(
+        user_id=206979918,
+        since=datetime(2026, 3, 1, 0, 0, 0, tzinfo=timezone.utc),
+        until=datetime(2026, 3, 10, 0, 0, 0, tzinfo=timezone.utc),
+    )
+    assert len(tracks) == 1
+    assert mock_sleep.call_count == 3
+
+
+@pytest.mark.asyncio
+@patch("ntr_fetcher.soundcloud.asyncio.sleep", return_value=None)
+async def test_fetch_likes_raises_when_all_requests_fail_500(mock_sleep, httpx_mock):
+    """When both fabricated cursor and cursorless fallback fail, the error propagates."""
+    httpx_mock.add_response(url="https://soundcloud.com", text=FAKE_HTML)
+    # 3 retries for fabricated cursor + 3 retries for cursorless fallback = 6 API calls
+    for _ in range(6):
+        httpx_mock.add_response(
+            url=re.compile(r"https://api-v2\.soundcloud\.com/users/206979918/likes.*"),
+            status_code=500,
+        )
+        httpx_mock.add_response(url="https://soundcloud.com", text=FAKE_HTML)
+    client = SoundCloudClient()
+    with pytest.raises(httpx.HTTPStatusError, match="500"):
+        await client.fetch_likes(
+            user_id=206979918,
+            since=datetime(2026, 3, 1, 0, 0, 0, tzinfo=timezone.utc),
+            until=datetime(2026, 3, 10, 0, 0, 0, tzinfo=timezone.utc),
+        )
+    assert mock_sleep.call_count == 6