Add admin request deletion with soft delete and cleanup

Implements admin ability to delete requests with soft delete, media file cleanup, and seeding-aware torrent management. Adds new API endpoint, frontend confirmation dialog, and request actions dropdown. Updates database schema with deletedAt and deletedBy fields, and ensures all queries filter out deleted requests. Documentation added for feature and user flow.

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

@@ -106,15 +106,18 @@ export async function processMonitorDownload(payload: MonitorDownloadPayload): P
       });
 
       // Get request with audiobook details
-      const request = await prisma.request.findUnique({
-        where: { id: requestId },
+      const request = await prisma.request.findFirst({
+        where: {
+          id: requestId,
+          deletedAt: null,
+        },
         include: {
           audiobook: true,
         },
       });
 
       if (!request || !request.audiobook) {
-        throw new Error('Request or audiobook not found');
+        throw new Error('Request or audiobook not found or deleted');
       }
 
       // Trigger organize files job (target path determined by database config)