Add Pushover notification support

- Add pushover_user_key and pushover_app_token columns to users table
- Add sendPushoverNotification function to notifications service
- Add Pushover test endpoint
- Add Pushover settings UI in Settings page
- User provides both User Key and App Token (self-hosted friendly)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -127,6 +127,7 @@ export interface NotificationSettings {
   telegram_configured: boolean;
   telegram_chat_id: string | null;
   discord_configured: boolean;
+  pushover_configured: boolean;
 }
 
 export const settingsApi = {
@@ -137,6 +138,8 @@ export const settingsApi = {
     telegram_bot_token?: string | null;
     telegram_chat_id?: string | null;
     discord_webhook_url?: string | null;
+    pushover_user_key?: string | null;
+    pushover_app_token?: string | null;
   }) => api.put<NotificationSettings & { message: string }>('/settings/notifications', data),
 
   testTelegram: () =>
@@ -145,6 +148,9 @@ export const settingsApi = {
   testDiscord: () =>
     api.post<{ message: string }>('/settings/notifications/test/discord'),
 
+  testPushover: () =>
+    api.post<{ message: string }>('/settings/notifications/test/pushover'),
+
   // AI Settings
   getAI: () =>
     api.get<AISettings>('/settings/ai'),