fix(sync): read a dark RSS feed through the API instead

YouTube's RSS goes dark per channel for hours (prod 2026-09-18/19: 393 of
396 feeds answered 404 for four hours, sticky per channel, so a retry does
not help), and nothing else read a followed channel's new uploads. A
channel whose feed has not answered for `rss_fallback_hours` (admin knob,
default 2, 0 = off) now gets one page of its uploads playlist through the
Data API (1 unit), at most once per window, yielding to the backfill
reserve. Spend is attributed to its own quota action.

backend/tests/test_followed_channels.py

@@ -27,7 +27,7 @@ def test_the_rss_poll_skips_channels_nobody_follows(db, make_user, monkeypatch):
     polled: list[str] = []
     monkeypatch.setattr(runner, "fetch_channel_feed", lambda cid: polled.append(cid) or [])
     monkeypatch.setattr(runner, "apply_rss_feed", lambda db, ch, entries: 0)
-    assert runner.run_rss_poll(db) == {"new": 0, "failed": 0}
+    assert runner.run_rss_poll(db) == {"new": 0, "failed": 0, "fallback": 0}
     assert sorted(polled) == ["UC_explored", "UC_sub"]