feat: admin app version mgmt + notification system

- New tables: app_config, notifications
- Admin views: App Version editor, Notification sender
- API: GET /api/notifications, POST /api/notifications/:id/read, POST /api/notifications/read-all
- App version now stored in DB, editable by super admin
- Notifications: send to all users or specific user
- Android notification models + API client
- Sidebar: App Version + Notifications links for super_admin

database/migrations/005_app_config_and_notifications.sql

@@ -0,0 +1,24 @@
+CREATE TABLE IF NOT EXISTS app_config (
+    `key` VARCHAR(64) PRIMARY KEY,
+    `value` TEXT NOT NULL,
+    updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+INSERT IGNORE INTO app_config (`key`, `value`) VALUES ('app_version', '1.0.1');
+INSERT IGNORE INTO app_config (`key`, `value`) VALUES ('apk_url', '/sysadmin.apk');
+INSERT IGNORE INTO app_config (`key`, `value`) VALUES ('force_update', '0');
+INSERT IGNORE INTO app_config (`key`, `value`) VALUES ('release_notes', '• Initial release\n• Server management\n• SSH terminal');
+
+CREATE TABLE IF NOT EXISTS notifications (
+    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    user_id INT UNSIGNED DEFAULT NULL,
+    title VARCHAR(255) NOT NULL,
+    message TEXT NOT NULL,
+    type ENUM('info','warning','success','error') NOT NULL DEFAULT 'info',
+    is_read TINYINT(1) NOT NULL DEFAULT 0,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    read_at DATETIME DEFAULT NULL,
+    INDEX idx_user_id (user_id),
+    INDEX idx_is_read (is_read),
+    INDEX idx_created_at (created_at)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;