Overhaul download retry pipeline: circuit breaker, escalating backoff, shelve logic

- Provider circuit breaker: track consecutive failures per provider with
  escalating cooldowns (30s/60s/120s/300s), auto-invalidate Mega-Debrid
  session on cooldown
- Escalating backoff: retry delays now scale up to 120s (was 30s max),
  unrestrict backoff exponential instead of linear 15s cap
- Shelve logic: after 15 consecutive failures, item pauses 5 min with
  counter halving for gradual recovery
- Periodic soft-reset: every 10 min, reset stale retry counters (>10 min
  queued) and old provider failures (>15 min), acts like mini-restart
- Mega-Debrid queue timeout: 90s wait limit in runExclusive to prevent
  cascade blocking behind stuck calls
- Provider-cooldown-aware retry delays: items wait for provider cooldown
  instead of retrying against broken service
- Fix: reconnect/package_toggle now persist retry counters (previously
  lost on interruption, defeating shelve logic)
- Mega-Debrid generate: tighter timeouts, progressive reload backoff,
  hoster retry limit (5x max)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/app-controller.ts

@@ -58,7 +58,8 @@ export class AppController {
       password: this.settings.megaPassword
     }));
     this.manager = new DownloadManager(this.settings, session, this.storagePaths, {
-      megaWebUnrestrict: (link: string, signal?: AbortSignal) => this.megaWebFallback.unrestrict(link, signal)
+      megaWebUnrestrict: (link: string, signal?: AbortSignal) => this.megaWebFallback.unrestrict(link, signal),
+      invalidateMegaSession: () => this.megaWebFallback.invalidateSession()
     });
     this.manager.on("state", (snapshot: UiSnapshot) => {
       this.onStateHandler?.(snapshot);