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/processors/direct-download.processor.ts

@@ -316,6 +316,7 @@ async function downloadFileWithProgress(
     let bytesDownloaded = 0;
     let lastLogTime = Date.now();
     let lastDbUpdateTime = Date.now();
+    let dbUpdatePending = false; // Guard against stacking unresolved DB updates
 
     response.data.on('data', (chunk: Buffer) => {
       bytesDownloaded += chunk.length;
@@ -332,18 +333,18 @@ async function downloadFileWithProgress(
         logger.info(`Download progress: ${percent}% (${(bytesDownloaded / (1024 * 1024)).toFixed(1)} MB, ${speedMBps.toFixed(2)} MB/s)`);
         lastLogTime = now;
 
-        // Update database with progress (non-blocking)
-        if (now - lastDbUpdateTime >= PROGRESS_UPDATE_INTERVAL_MS) {
+        // Update database with progress (non-blocking, at most 1 in-flight at a time)
+        if (now - lastDbUpdateTime >= PROGRESS_UPDATE_INTERVAL_MS && !dbUpdatePending) {
           lastDbUpdateTime = now;
+          dbUpdatePending = true;
 
-          // Non-blocking update - fire and forget
           prisma.request.update({
             where: { id: tracking.requestId },
             data: {
               progress: Math.min(percent, 99), // Cap at 99% until fully complete
               updatedAt: new Date(),
             },
-          }).catch(() => {}); // Ignore errors during progress update
+          }).catch(() => {}).finally(() => { dbUpdatePending = false; });
         }
       }
     });