security: add PHP session auth guard for admin panel (item #2, CRITICAL)

- lib/AdminAuth.php: new class with requireLogin(), login(), logout(),
  isAuthenticated(); starts session with hardened cookie params
  (HttpOnly, SameSite=Strict, Secure, Path=/admin) — also resolves
  item #8 (session cookie hardening)
- requireLogin() auto-authenticates from nginx Basic Auth credentials
  ($_SERVER['PHP_AUTH_PW']) so the user only sees one browser prompt;
  falls back to /admin/login.php if the proxy is absent/misconfigured
- config/admin_credentials.php: gitignored credential store; define
  ADMIN_PASSWORD_HASH with a bcrypt hash to enable PHP auth
- config/admin_credentials.example.php: template for the above
- config/bootstrap.php: auto-loads admin_credentials.php if present
- .gitignore: exclude config/admin_credentials.php
- public/admin/login.php: fallback login form (shown only when nginx
  Basic Auth is bypassed / proxy absent)
- public/admin/logout.php: session destruction + redirect to login
- All 7 admin PHP files: replace session_start() with
  AdminAuth::requireLogin() (defence-in-depth behind nginx Basic Auth)
- public/admin/inc/head.php: Déconnexion button when ADMIN_PASSWORD_HASH
  is defined
- nginx/PHP_AUTH_LAYER.md: documents dual-auth architecture, UX flow,
  and setup instructions
- docs/TODO.SECURITY.md: items #2 and #8 moved to Resolved; priority
  order updated (all CRITICAL done)

public/admin/actions/formulaire.php

@@ -1,15 +1,15 @@
 <?php // formulaire.php
 // Bootstrap application
 require_once __DIR__ . "/../../config/bootstrap.php";
-
+require_once __DIR__ . '/../../lib/AdminAuth.php';
 
 // Configure error reporting
 ini_set('display_errors', 0);
 ini_set('log_errors', 1);
 ini_set('error_log', 'error.log');
 
-// Start session for CSRF protection
-session_start();
+// PHP-level auth guard (defence-in-depth behind nginx Basic Auth)
+AdminAuth::requireLogin();
 
 // Verify CSRF token
 if (!isset($_POST['csrf_token']) || !isset($_SESSION['csrf_token']) ||