fix(youtube): the scrape search never pins a stale key, and an unknown shape is an error

A failed config fetch or a keyless consent page cached the fallback InnerTube key for the life
of the process; once it went stale, search returned nothing with no error. The fallback is now
used for that call only. A 200 JSON without a result frame returned an empty page that read as
no results; it is a ScrapeError now. Parser, cursor and Shorts-probe tests added.

backend/tests/test_shorts_probe.py

@@ -0,0 +1,44 @@
+"""youtube/shorts.probe_is_short — three answers, and the third is not the second (E32 S2: 33% covered).
+
+A Short answers 200 at /shorts/<id>; a regular video redirects to /watch. Anything else — an error
+page, a network failure — is "undetermined, ask again", never "not a Short": that would file a Short
+in the normal feed for good.
+"""
+
+import httpx
+import pytest
+
+from app.youtube import shorts
+
+
+def _client(respond) -> httpx.Client:
+    return httpx.Client(transport=httpx.MockTransport(respond))
+
+
+@pytest.mark.parametrize(
+    ("status", "expected"),
+    [(200, True), (301, False), (302, False), (303, False), (307, False), (308, False)]
+    + [(404, None), (429, None), (500, None)],
+)
+def test_the_status_decides_short_regular_or_undetermined(status, expected):
+    seen: list[httpx.Request] = []
+
+    def respond(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(status, headers={"location": "https://www.youtube.com/watch?v=x"})
+
+    assert shorts.probe_is_short(_client(respond), "abc123") is expected
+    assert str(seen[0].url) == "https://www.youtube.com/shorts/abc123"
+    assert len(seen) == 1  # a redirect is the answer, never followed
+
+
+def test_a_network_failure_is_undetermined_not_regular():
+    def respond(request: httpx.Request) -> httpx.Response:
+        raise httpx.ConnectTimeout("slow", request=request)
+
+    assert shorts.probe_is_short(_client(respond), "abc123") is None
+
+
+def test_the_client_skips_the_consent_interstitial():
+    with shorts.make_client() as c:
+        assert "SOCS=" in c.headers["cookie"]