feat: add user language preference endpoint (#86)

[kevalyq]

Summary

Implements user language preference functionality to allow users to set their preferred UI language (English or German) in their profile.

Changes

Database

• ✅ Add preferred_locale column to users table (VARCHAR(5), nullable)
• ✅ Migration: 2025_11_16_192506_add_preferred_locale_to_users_table

API

• ✅ New endpoint: PATCH /v1/me/language
• ✅ Request validation via UpdateUserLanguageRequest
• ✅ Supported locales: en (English), de (German), null (reset to default)
• ✅ Controller method: AuthController@updateLanguage

Models

• ✅ Update User model with preferred_locale in fillable
• ✅ Update UserFactory to include preferred_locale
• ✅ PHPDoc annotations updated

Tests

• ✅ 8 comprehensive feature tests (TDD approach)
• ✅ All 403 tests passing
• ✅ PHPStan level max: 0 errors
• ✅ Laravel Pint: Clean
• ✅ REUSE 3.3: Compliant

Testing

# Run tests
ddev exec php artisan test --filter=UserLanguagePreferenceTest

# All tests
ddev exec php artisan test

# Static analysis
ddev exec vendor/bin/phpstan analyse --memory-limit=512M

# Code style
ddev exec vendor/bin/pint

Acceptance Criteria (from #86)

• Database column preferred_locale added to users table
• API endpoint PATCH /v1/me/language created
• Validation: Only en, de, or null accepted
• Empty string treated as null (reset preference)
• Endpoint only updates authenticated user's preference
• Unauthenticated requests return 401
• Comprehensive test coverage (8 tests)

Documentation

• ✅ CHANGELOG.md updated with new feature

Follow-up

Frontend implementation (#141) can now use this endpoint to persist user language preferences.

Closes #86

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull Request Overview

This PR implements user language preference functionality allowing users to set and persist their preferred UI language (English or German) via a new API endpoint. The implementation follows TDD principles with comprehensive test coverage.

Key Changes

• Database schema extended with preferred_locale column in the users table
• New PATCH endpoint /v1/me/language for updating language preferences
• Comprehensive validation ensuring only supported locales (en, de, or null) are accepted

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/Feature/Controllers/Api/V1/UserLanguagePreferenceTest.php	Adds 8 comprehensive feature tests covering happy paths, validation, and authentication
routes/api.php	Registers new language preference endpoint in v1 API routes
database/migrations/2025_11_16_192506_add_preferred_locale_to_users_table.php	Adds nullable preferred_locale column to users table
database/factories/UserFactory.php	Updates factory to include preferred_locale with default null value
app/Models/User.php	Adds preferred_locale to fillable array and PHPDoc annotations
app/Http/Requests/UpdateUserLanguageRequest.php	Validates locale input using present, nullable, and Rule::in()
app/Http/Controllers/AuthController.php	Implements updateLanguage() method to handle language preference updates
CHANGELOG.md	Documents the new user language preference feature