Add configurable registration modes: open, code-based, and closed

[Copilot]

Implements three registration modes controlled via REGISTRATION_MODE constant in /assets/config/config.php:

define('REGISTRATION_MODE', 'open');   // Default: anyone can register
define('REGISTRATION_MODE', 'code');   // Requires unique invitation code
define('REGISTRATION_MODE', 'closed'); // No new registrations

Changes

Configuration

• Added REGISTRATION_MODE constant to assets/config/config.php
• Default value: 'open' (preserves existing behavior)

Authentication Flow

• Modified auth/callback.php to check registration mode before user creation
• In code mode: redirect new users to login page for code entry, validate code, mark as used
• In closed mode: redirect to login page with error message
• In open mode: unchanged behavior (auto-create with default role)

Admin Interface

• Created benutzer/registration-codes.php for code management
• Requires admin or users.manage permission
• Generate 16-character random codes, view usage history, delete unused codes

User Interface

• Updated login.php to handle code entry directly on the login page
• Login page dynamically shows code input form when required (?require_code=1)
• Displays mode-specific messages and error handling
• Session-based error messages for better user experience

Database

• New table intra_registration_codes tracks codes, creator, and usage
• Migration: create_intra_registration_codes_02112025.php

Security

• XSS protection via htmlspecialchars() on error messages
• Prevent deletion of used codes via WHERE is_used = 0
• Optimized existence checks with SELECT 1 instead of full row fetch

User Flow

All registration interactions happen on the main login page:

• Open mode: Standard Discord login button
• Code mode: After Discord auth, new users see code entry form inline
• Closed mode: New users see error message on login page

Fixes #123

Original prompt

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This section details on the original issue you should resolve

<issue_title>Config-Variable für Login</issue_title>
<issue_description>1. Registrierung für jeden möglich
2. Registrierung nur mit einzigartigem Code (mit eigener Berechtigung erstellbar)
3. Keine Registrierung für neue Personen möglich</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Config-Variable für Login #123

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[itshypax]

@copilot When registration mode is "closed" redirect the user back to the login page with the warning/error message. Also handle entering the registration code (if required) on the normal login page.

[itshypax]

Für die Landingpage bzw. Wiki gut, nicht im Repo hier

[Copilot]

@copilot When registration mode is "closed" redirect the user back to the login page with the warning/error message. Also handle entering the registration code (if required) on the normal login page.

Done in commit 6fdf773.

Changes made:

1. When registration mode is "closed", users are now redirected back to the login page with an error message instead of showing a plain error page
2. Registration code entry is now handled directly on the login page (no separate page)
3. The login page dynamically shows the code input form when required

UI Flow:

• Normal login: Shows standard Discord login button
• Code mode: After Discord auth, new users are redirected to login page with ?require_code=1 parameter, which displays the code entry form
• Closed mode: New users attempting to register are redirected back to login page with error message

The screenshot above shows all scenarios including the code entry form integrated into the login page.