feat(search): zero-quota InnerTube scrape source + search_source toggle

Live YouTube search can now use YouTube's internal InnerTube endpoint instead
of search.list, materialising results through the same enrich/provenance path
at zero API quota (search.list costs 100 units/page; scrape costs nothing, only
the cheap shared videos.list enrich is charged).

- youtube/search_scrape.py: InnerTube search returning the same page shape as
  YouTubeClient.search_videos (items + continuation cursor); SOCS consent cookie,
  videoRenderer walk (channels/playlists/Shorts shelves naturally excluded),
  type:video filter, cached InnerTube key/version with constant fallbacks.
- routes/search.py: admin-selectable source (search_source, default scrape).
  Scrape path skips the 100-unit budget pre-check and logs a zero-cost search
  event so the per-user daily cap still counts it; api path unchanged. Response
  carries the active source.
- sysconfig/config: new search_source key (scrape|api).
- quota.log_action: record a zero-cost action event for per-user rate limits.

backend/app/sysconfig.py

@@ -40,6 +40,8 @@ SPECS: tuple[ConfigSpec, ...] = (
     ConfigSpec("quota_daily_budget", "int", "quota", "quota_daily_budget", min=1, max=1_000_000),
     ConfigSpec("backfill_quota_reserve", "int", "quota", "backfill_quota_reserve", min=0, max=1_000_000),
     ConfigSpec("search_daily_limit_per_user", "int", "quota", "search_daily_limit_per_user", min=0, max=100_000),
+    # Live-search source: "scrape" (InnerTube, zero API quota) or "api" (search.list, 100u/page).
+    ConfigSpec("search_source", "str", "quota", "search_source"),
     # --- Backfill (recent-first first pass per channel) ---
     ConfigSpec("backfill_recent_max_videos", "int", "backfill", "backfill_recent_max_videos", min=1, max=100_000),
     ConfigSpec("backfill_recent_max_days", "int", "backfill", "backfill_recent_max_days", min=1, max=36_500),