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.

documentation/TABLEOFCONTENTS.md

@@ -62,6 +62,7 @@
 ## Admin Features
 - **Dashboard (metrics, downloads, requests)** → [admin-dashboard.md](admin-dashboard.md)
 - **Jobs management UI** → [backend/services/scheduler.md](backend/services/scheduler.md)
+- **Request deletion (soft delete, seeding awareness)** → [admin-features/request-deletion.md](admin-features/request-deletion.md)
 
 ## Deployment
 - **Docker Compose setup (multi-container)** → [deployment/docker.md](deployment/docker.md)
@@ -78,6 +79,7 @@
 **"What's the database schema?"** → [backend/database.md](backend/database.md)
 **"How does authentication work?"** → [backend/services/auth.md](backend/services/auth.md)
 **"How do I change the admin password?"** → [settings-pages.md](settings-pages.md), [backend/services/auth.md](backend/services/auth.md)
+**"How do I delete requests?"** → [admin-features/request-deletion.md](admin-features/request-deletion.md)
 **"How do I deploy?"** → [deployment/docker.md](deployment/docker.md) (multi-container), [deployment/unified.md](deployment/unified.md) (all-in-one)
 **"How do I use the unified container?"** → [deployment/unified.md](deployment/unified.md)
 **"OAuth redirects to localhost / PUBLIC_URL not working"** → [backend/services/environment.md](backend/services/environment.md)