feat: licence page, admin pages editor, license types, gradient card placeholders, latest-year home view

- Feature 1: public /licence.php fetches 'licenses' page from DB, renders Markdown
- Feature 1: nav.php adds 'Licence' link with active state
- Feature 2: Database::getPage(), savePage(), getAllPages() methods
- Feature 2: bundled src/Parsedown.php (MIT, zero-dependency)
- Feature 2: apropos.php now renders 'about' page content from DB via Parsedown
- Feature 2: admin/pages.php (list) + admin/pages-edit.php (EasyMDE editor)
- Feature 2: admin/actions/page.php (auth+CSRF+validation+save)
- Feature 2: admin/head.php adds 'Pages statiques' nav link
- Feature 3: storage/schema.sql seeds 8 CC license types
- Feature 3: storage/migrations/003_seed_license_types.sql (applied to live DB)
- Feature 3: Database::getLicenseTypes() / getAllLicenseTypes()
- Feature 3: admin/add.php + formulaire.php: license_id field on add form
- Feature 3: admin/edit.php: license_id field on edit form with raw FK lookup
- Feature 3: tfe.php: shows 'Licence :' meta row when non-null
- Feature 6: main.css: .card__media--gradient styles
- Feature 6: index.php: deterministic HSL gradient placeholder cards
- Feature 6: Database::getLatestYearTheses() + getLatestPublishedYear()
- Feature 6: index.php default home = random latest-year theses with info label

public/admin/actions/page.php

@@ -0,0 +1,35 @@
+<?php
+require_once __DIR__ . "/../../config/bootstrap.php";
+require_once __DIR__ . '/../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+// CSRF check
+if (!isset($_POST['csrf_token']) || !isset($_SESSION['csrf_token']) ||
+    !hash_equals($_SESSION['csrf_token'], $_POST['csrf_token'])) {
+    die("Erreur de sécurité : token invalide.");
+}
+
+$allowedSlugs = ['about', 'licenses', 'charte', 'contact'];
+$slug = $_POST['slug'] ?? '';
+if (!in_array($slug, $allowedSlugs)) {
+    die("Slug invalide.");
+}
+
+$content = $_POST['content'] ?? '';
+if (strlen($content) > 65535) {
+    die("Contenu trop long (max 65 535 caractères).");
+}
+
+require_once __DIR__ . '/../../src/Database.php';
+
+try {
+    $db = new Database();
+    $db->savePage($slug, $content);
+    $_SESSION['success'] = "Page «" . $slug . "» mise à jour avec succès.";
+} catch (Exception $e) {
+    error_log("Page save error: " . $e->getMessage());
+    die("Erreur lors de la sauvegarde : " . htmlspecialchars($e->getMessage()));
+}
+
+header('Location: /admin/pages.php');
+exit;