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

templates/nav.php

@@ -1,7 +1,7 @@
 <?php
 // nav.php — shared public navigation bar
 // Usage: include this partial from any public page
-// Provide $currentPage variable to mark active links (optional)
+// Provide $currentNav variable to mark active links (optional)
 $_navCurrent = $currentNav ?? '';
 ?>
 <nav class="site-nav">
@@ -9,6 +9,8 @@ $_navCurrent = $currentNav ?? '';
     <div class="site-nav__links">
         <a class="site-nav__link <?= ($_navCurrent === 'repertoire') ? 'site-nav__link--active' : '' ?>"
            href="/search.php">Répertoire</a>
+        <a class="site-nav__link <?= ($_navCurrent === 'licence') ? 'site-nav__link--active' : '' ?>"
+           href="/licence.php">Licence</a>
     </div>
     <a class="site-nav__right <?= ($_navCurrent === 'apropos') ? 'site-nav__link--active' : '' ?>"
        href="/apropos.php">À Propos</a>