Add DB pooling, throttling and monitor backoff

Add connection pool params to DATABASE_URL and configure Prisma to use the pooled URL (connection_limit=20, pool_timeout=30) to reduce connection exhaustion. Introduce safeguards and throttling across processors: limit in-flight progress DB updates in direct-download, add short delays when processing RSS, retry-failed-imports, and retry-missing-torrents, and stagger scheduler triggers to avoid bursts. Implement adaptive monitor-download polling with stallCount/lastProgress and exponential backoff, and thread these fields through JobQueueService (including reduced worker concurrency for several queues). Batch audiobook enrichment queries to small parallel batches to limit DB load. Update tests to reflect new monitor payload parameters. Overall intent: reduce DB connection pool pressure and smooth load spikes during startup and heavy processing.

src/lib/services/scheduler.service.ts

@@ -491,6 +491,9 @@ export class SchedulerService {
         if (this.isJobOverdue(job)) {
           logger.info(`Job "${job.name}" is overdue, triggering now...`);
           await this.triggerJobNow(job.id);
+
+          // Stagger triggers to avoid connection pool burst on startup
+          await new Promise(resolve => setTimeout(resolve, 500));
         }
       } catch (error) {
         logger.error(`Failed to trigger overdue job "${job.name}"`, { error: error instanceof Error ? error.message : String(error) });