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/app/config.py

@@ -119,6 +119,9 @@ class Settings(BaseSettings):
     # Recent-first backfill: how far back to fetch on the first pass per channel.
     backfill_recent_max_videos: int = 100
     backfill_recent_max_days: int = 365
+    # A channel whose RSS feed has not answered for this many hours gets its newest uploads read
+    # through the Data API instead (1 unit), at most once per this many hours. 0 = never.
+    rss_fallback_hours: int = 2
 
     # Shorts are confirmed by probing youtube.com/shorts/<id>. Only videos at or below
     # this duration (and not livestreams) are probed; longer videos are never Shorts.