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.

documentation/settings-pages.md

@@ -70,6 +70,7 @@ src/app/admin/settings/
 4. **Download Client** - Type, URL, credentials (masked)
 5. **Paths** - Download + media directories, audiobook organization template, metadata tagging toggle, chapter merging toggle
 6. **BookDate** - AI provider, API key (encrypted), model selection, library scope, custom prompt, swipe history
+7. **Notifications** - Multiple backends (Discord, Pushover), event subscriptions, test functionality
 
 ## Audible Region
 
@@ -300,3 +301,38 @@ src/app/admin/settings/
   - Allow saving indexer config changes without re-testing connection
   - Button text adapts: "Test Connection" vs "Refresh Indexers"
 - Behavior: Natural workflow - see current settings, modify indexers, save immediately
+
+## Notifications
+
+**Purpose:** Configure notification backends to receive alerts for audiobook request events.
+
+**Configuration:**
+- Multiple backends per type (Discord, Pushover)
+- Per-backend event subscriptions (4 events)
+- Encrypted sensitive values (webhook URLs, API keys)
+- Enable/disable toggle per backend
+
+**UI (NotificationsTab):**
+- Type selector cards: Discord (indigo "D"), Pushover (blue "P")
+- Grid layout for configured backends (3 columns)
+- Card shows: type icon, name, enabled status, event count
+- Edit/delete actions per card
+
+**Modal (NotificationConfigModal):**
+- Type-specific forms (Discord: webhook/username/avatar, Pushover: keys/priority)
+- Event subscription checkboxes (4 events)
+- Enable/disable toggle
+- Test button (sends sample notification)
+- Password masking for sensitive values
+
+**Event Types:**
+- Request Pending Approval - Admin approval required
+- Request Approved - Approved (manual or auto)
+- Request Available - Available in library
+- Request Error - Failed at any stage
+
+**Validation:**
+- Name required
+- Discord: webhook URL required
+- Pushover: user key + app token required
+- At least one event selected