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/scan-plex.processor.ts

@@ -140,7 +140,10 @@ export async function processScanPlex(payload: ScanPlexPayload): Promise<any> {
     // 5. Match downloaded requests against library
     await logger?.info(`Checking for downloaded requests to match...`);
     const downloadedRequests = await prisma.request.findMany({
-      where: { status: 'downloaded' },
+      where: {
+        status: 'downloaded',
+        deletedAt: null,
+      },
       include: { audiobook: true },
       take: 50, // Limit to prevent overwhelming
     });