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/assets/main.css

@@ -91,6 +91,41 @@ html, body {
   font-size: 2rem;
 }
 
+.card__media--gradient {
+  width: 100%;
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  padding: 1rem;
+  text-align: center;
+  box-sizing: border-box;
+}
+
+.card__gradient-author {
+  color: #fff;
+  font-size: 0.75rem;
+  opacity: .85;
+  margin-bottom: .25rem;
+  display: block;
+}
+
+.card__gradient-title {
+  color: #fff;
+  font-size: 0.85rem;
+  font-weight: 600;
+  display: -webkit-box;
+  -webkit-line-clamp: 3;
+  -webkit-box-orient: vertical;
+  overflow: hidden;
+}
+
+/* Latest-year label */
+.home-latest-label {
+  font-style: italic;
+}
+
 .card__info {
   padding: 0.55rem 0.5rem 0.65rem;
   font-size: 0.88rem;