🔒 Fix hardcoded database credentials and SQL injection vulnerabilities

[tsainez]

🎯 What: Hardcoded database credentials were found in php/cookies_and_access_control/doLogin.php and php/cookies_and_access_control/addUser.php. In addition, SQL injection vulnerabilities were present in both files due to direct concatenation of user input into SQL queries.

⚠️ Risk: Hardcoded credentials in source code can easily be leaked through version control, putting the database at risk of unauthorized access. Furthermore, the direct concatenation of user input into queries exposes the application to SQL injection attacks, which could allow malicious users to extract or alter data.

🛡️ Solution: Modified the files to retrieve database connection details (host, db, username, password) securely via environment variables using getenv(). Additionally, updated the PDO database interactions to use parameterized prepared statements, completely mitigating the SQL injection risks. Fixed a minor syntax error in addUser.php's INSERT statement where single quotes were incorrectly used around table and column names.

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PR created automatically by Jules for task 14814758747756920596 started by @tsainez

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