Add notification system with admin UI and backend

Introduces a full notification system with support for Discord and Pushover backends, event triggers, and message formatting. Adds backend services, processors, and API endpoints for managing notifications, as well as a new Notifications tab in the admin settings UI. Updates documentation, database schema, and tests to cover notification features and approval workflow improvements. Also changes project license from MIT to AGPL v3.

prisma/schema.prisma

@@ -210,6 +210,7 @@ model Request {
   progress         Int       @default(0) // 0-100
   priority         Int       @default(0)
   errorMessage     String?   @map("error_message") @db.Text
+  selectedTorrent  Json?     @map("selected_torrent") // Pre-selected torrent from interactive search (stored when awaiting approval)
   searchAttempts   Int       @default(0) @map("search_attempts")
   downloadAttempts Int       @default(0) @map("download_attempts")
   importAttempts   Int       @default(0) @map("import_attempts")
@@ -420,3 +421,17 @@ model BookDateSwipe {
   @@index([recommendationId])
   @@map("bookdate_swipes")
 }
+
+model NotificationBackend {
+  id        String   @id @default(uuid())
+  type      String   // 'discord' | 'pushover' | 'email' | 'slack' | 'telegram' | 'webhook'
+  name      String   // User-friendly label
+  config    Json     // Type-specific config (encrypted sensitive values)
+  events    Json     @default("[]") // Array of event strings
+  enabled   Boolean  @default(true)
+  createdAt DateTime @default(now()) @map("created_at")
+  updatedAt DateTime @updatedAt @map("updated_at")
+
+  @@index([enabled])
+  @@map("notification_backends")
+}