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

storage/migrations/003_seed_license_types.sql

@@ -0,0 +1,19 @@
+-- Migration 003: Seed license_types table
+-- Safe to run on existing databases (INSERT OR IGNORE)
+
+CREATE TABLE IF NOT EXISTS license_types (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    name TEXT NOT NULL UNIQUE,
+    description TEXT,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+INSERT OR IGNORE INTO license_types (name) VALUES
+    ('CC BY 4.0'),
+    ('CC BY-SA 4.0'),
+    ('CC BY-ND 4.0'),
+    ('CC BY-NC 4.0'),
+    ('CC BY-NC-SA 4.0'),
+    ('CC BY-NC-ND 4.0'),
+    ('Tous droits réservés'),
+    ('Domaine public');