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/services/notification/providers/discord.provider.ts

@@ -71,7 +71,7 @@ export class DiscordProvider implements INotificationProvider {
     ];
 
     if (message) {
-      fields.push({ name: isIssue ? 'Reason' : 'Error', value: message, inline: false });
+      fields.push({ name: meta.messageLabel ?? 'Error', value: message, inline: false });
     }
 
     return {