feat: add user preference management system

[Nimatstar]

Summary

Implements a user preference management system allowing users to manage notification preferences, language, and theme settings.

Changes

• src/user-preferences/entities/user-preference.entity.ts - UserPreference entity with theme, language, and notification preference columns
• src/user-preferences/dto/update-user-preference.dto.ts - Validated DTO for partial preference updates
• src/user-preferences/user-preferences.service.ts - Service: get (with auto-create defaults), update, and reset preferences
• src/user-preferences/user-preferences.controller.ts - REST endpoints under users/:userId/preferences (GET, PATCH, DELETE)
• src/user-preferences/user-preferences.module.ts - Module wiring

Acceptance Criteria

• Preferences table created (user_preferences)
• Settings API endpoints (GET/PATCH/DELETE /users/:userId/preferences)
• Notification preferences (emailNotifications, pushNotifications, inAppNotifications, marketingEmails, courseUpdates, weeklyDigest)
• Language selection (en, fr, es, de, ar)

closes #552

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