feat: upgrade project to Symfony 5.4 and PHP 8.1 with complete test coverage

[Mike031289]

Description

This Pull Request marks the complete modernization of the ToDo & Co application, migrating from the legacy stack (Symfony 3 / PHP 5.5) to a modern, maintainable, and secure architecture under Symfony 5.4 and PHP 8.1.

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Key Changes Implemented

1. Technical Migration & Upgrade

• Framework & PHP: Upgraded to Symfony 5.4 (LTS) with PHP 8.1 compatibility.
• Project Restructuring: Adopted modern Symfony directory layout (removed legacy app/ and web/ directories in favor of config/, public/, and templates/).
• Quality & Analysis: Cleaned up code using Rector and ensured compliance with coding standards (PSR-12).

2. Security & Access Control

• Voters: Implemented a custom TaskVoter to secure resource access and prevent unauthorized modifications (strict deletion rules for standard users and administrators regarding anonymous tasks).
• CLI Commands: Added a maintenance command to safely migrate orphaned tasks to the system anonyme user account.

3. Testing & Quality Assurance (QA)

• PHPUnit 9.6 Suite: The entire suite of functional and unit tests has been updated and validated (35 tests / 94 assertions passing successfully).
• Documentation: Fully updated the README.md, technical security documentation (doc/TECHNICAL_SECURITY.md), and integrated the final quality audit report (v1.0.0).

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✅ Checklist

• All unit and functional tests pass successfully (vendor/bin/phpunit).
• Code style complies with PSR-12 standards.
• Documentation and audit reports are up to date.

[codacy-production]

Up to standards ✅

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