feat: Add persistent global notification settings for admin panel

Implement database-backed notification settings that persist across restarts:

Backend changes:
- Updated DashboardService to read/write notification settings from database
- Added 6 notification settings keys to dbSettingsMap:
  * enable_email_notifications (boolean)
  * enable_push_notifications (boolean)
  * admin_notifications (boolean)
  * error_alerts (boolean)
  * new_user_alerts (boolean)
  * system_health_alerts (boolean)
- Settings are now retrieved from database with fallback defaults

Database:
- Seeded 6 default notification settings in settings table
- All notification toggles default to 'true'
- Settings persist across server restarts

Frontend:
- Admin settings page at /settings already configured
- Notifications tab contains all 6 toggle switches
- Settings are loaded from GET /api/v1/admin/dashboard/settings
- Settings are saved via POST /api/v1/admin/dashboard/settings

API Endpoints (already existed, now enhanced):
- GET /api/v1/admin/dashboard/settings - Returns all settings including notifications
- POST /api/v1/admin/dashboard/settings - Persists notification settings to database

Files modified:
- maternal-app-backend/src/modules/admin/dashboard/dashboard.service.ts

Benefits:
✅ Global notification settings are now persistent
✅ Admin can control email/push notifications globally
✅ Admin can configure alert preferences
✅ Settings survive server restarts and deployments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-app-backend/src/database/entities/push-subscription.entity.ts

@@ -0,0 +1,62 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  ManyToOne,
+  JoinColumn,
+  CreateDateColumn,
+  UpdateDateColumn,
+  Index,
+} from 'typeorm';
+import { User } from './user.entity';
+
+@Entity('push_subscriptions')
+@Index('idx_push_subs_user_id', ['userId'])
+@Index('idx_push_subs_active', ['isActive'], { where: 'is_active = true' })
+export class PushSubscription {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ name: 'user_id', type: 'varchar', length: 20 })
+  userId: string;
+
+  @Column({ type: 'text', unique: true })
+  endpoint: string;
+
+  @Column({ type: 'text' })
+  p256dh: string;
+
+  @Column({ type: 'text' })
+  auth: string;
+
+  @Column({ name: 'user_agent', type: 'text', nullable: true })
+  userAgent: string | null;
+
+  @Column({ name: 'device_type', type: 'varchar', length: 20, nullable: true })
+  deviceType: string | null;
+
+  @Column({ type: 'varchar', length: 50, nullable: true })
+  browser: string | null;
+
+  @Column({ name: 'is_active', type: 'boolean', default: true })
+  isActive: boolean;
+
+  @Column({ name: 'last_error', type: 'text', nullable: true })
+  lastError: string | null;
+
+  @Column({ name: 'failed_attempts', type: 'int', default: 0 })
+  failedAttempts: number;
+
+  @Column({ name: 'last_success_at', type: 'timestamp', nullable: true })
+  lastSuccessAt: Date | null;
+
+  @CreateDateColumn({ name: 'created_at', type: 'timestamp' })
+  createdAt: Date;
+
+  @UpdateDateColumn({ name: 'updated_at', type: 'timestamp' })
+  updatedAt: Date;
+
+  @ManyToOne(() => User, { onDelete: 'CASCADE' })
+  @JoinColumn({ name: 'user_id' })
+  user?: User;
+}