feat: add On Grab notification event

Adds request_grabbed event that fires when a torrent/NZB is successfully
handed off to the configured download client, filling the gap between
request_approved (pre-search) and request_available (fully imported).

- Add request_grabbed to NOTIFICATION_EVENTS with titleByRequestType
  (Audiobook Grabbed / Ebook Grabbed), info severity, Details messageLabel
- Add NotificationEventConfig interface and update getEventMeta() return
  type to expose messageLabel to all providers without TypeScript errors
- Add messageLabel: 'Reason' to issue_reported event
- Fix all 4 providers (Discord, ntfy, Pushover, Apprise) to derive message
  field label from meta.messageLabel ?? 'Error' instead of hardcoded
  isIssue ternary — prevents grab details showing as Error
- Trigger request_grabbed in download-torrent.processor.ts after
  client.addDownload() succeeds; message carries torrent title, indexer,
  and download client name; requestType sourced from request.type
- Update notifications.md documentation

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

src/lib/processors/download-torrent.processor.ts

@@ -103,8 +103,32 @@ export async function processDownloadTorrent(payload: DownloadTorrentPayload): P
 
     logger.info(`Created download history record: ${downloadHistory.id}`);
 
-    // Trigger monitor download job with initial delay
+    // Send grab notification
+    const requestWithUser = await prisma.request.findUnique({
+      where: { id: requestId },
+      include: {
+        user: { select: { plexUsername: true } },
+      },
+    });
+
     const jobQueue = getJobQueueService();
+
+    if (requestWithUser) {
+      const grabMessage = `${torrent.title} via ${torrent.indexer} (${client.clientType})`;
+      await jobQueue.addNotificationJob(
+        'request_grabbed',
+        requestId,
+        audiobook.title,
+        audiobook.author,
+        requestWithUser.user.plexUsername || 'Unknown User',
+        grabMessage,
+        requestWithUser.type
+      ).catch((error) => {
+        logger.error('Failed to queue grab notification', { error: error instanceof Error ? error.message : String(error) });
+      });
+    }
+
+    // Trigger monitor download job with initial delay
     await jobQueue.addMonitorJob(
       requestId,
       downloadHistory.id,