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.

frontend/src/i18n/locales/en/scheduler.json

@@ -76,7 +76,7 @@
     "plex_sync": "Mirrors the enabled Plex libraries into the app's catalogue — the metadata behind Plex browsing, search, filters and the info pages (genres, cast, ratings, artwork). If it stops, newly added or changed Plex titles don't appear until the next sync.",
     "plex_watch_sync": "Pulls recent Plex watch changes (finished episodes, resume positions) into Siftlode and re-pushes any local watch changes that didn't reach Plex. The cheap two-way keep-in-sync between full reconciles. If it stops, watched/resume made in the Plex app takes longer to show here.",
     "plex_watch_reconcile": "A full watch-state reconcile that rescans every library — catches what the incremental pass can't, notably an episode un-marked as watched on the Plex side. Heavier, so it runs about once a day. If it stops, un-watches done in Plex aren't mirrored back.",
-    "audit_gc": "Prunes admin audit-log entries older than the configured retention window (Configuration → Audit log). If it stops, the audit log just keeps growing — harmless but unbounded.",
+    "audit_gc": "Prunes admin audit-log entries and YouTube quota history older than their retention windows (Configuration → Audit log, Quota). If it stops, both just keep growing — harmless but unbounded.",
     "log_gc": "Deletes stored log events older than the retention window (Configuration → Diagnostics). Runs hourly so nothing outlives the promised limit by more than an hour.",
     "tmdb_enrich": "Enriches the mirrored Plex catalogue from TMDB (keywords, crew, budget/revenue, TMDb score, recommendations, artwork), matched by each title's canonical id and served from the app's own database. Optional — a no-op until a TMDB API key is set. If it stops, new Plex titles just miss their TMDB extras until it runs again.",
     "people_enrich": "Makes the people in the Plex library local: fetches each person's biography from TMDB and caches their portrait, resized through Plex, the most-credited people first. A person page and a cast strip then open from the app's own data. The TMDB half does nothing without a TMDB API key, the portrait half nothing without Plex. If it stops, a person page fetches its own bio when opened, a moment later."
@@ -97,7 +97,7 @@
     "plex_watch_sync": "Plex watch sync (incremental)",
     "plex_watch_reconcile": "Plex watch reconcile (full)",
     "tmdb_enrich": "TMDB enrichment (Plex)",
-    "audit_gc": "Audit-log cleanup",
+    "audit_gc": "Audit-log and quota-history cleanup",
     "log_gc": "Log cleanup",
     "people_enrich": "People details and portraits (Plex)"
   },