fix(quota): prune quota history past a retention window

quota_events had no retention: one row per API spend plus a zero-unit row per live search, kept
forever. The daily history cleanup (audit_gc) now also prunes it past quota_retention_days (a new
Quota setting, default 365 so the admin usage view's longest window stays whole; 0 = forever). The
floor of two days keeps today's rows, which the per-user search cap counts.

backend/app/config.py

@@ -260,6 +260,9 @@ class Settings(BaseSettings):
     # Admin audit log: how long to keep rows (days; 0 = keep forever) + how often the GC runs.
     audit_retention_days: int = 180
     audit_gc_minutes: int = 1440
+    # YouTube quota history (quota_events): days kept (0 = forever). A year, so the admin usage view's
+    # longest window (365 days) stays whole; the same daily cleanup job prunes it.
+    quota_retention_days: int = 365
     # On-disk layout: "plex" (Channel/Season YYYY/… [id].ext + .nfo/poster) or "flat".
     download_layout: DownloadLayout = "plex"
     # Default SponsorBlock (skip sponsor segments) for new custom profiles.