feat: admin tag management, maintenance mode, TFE visibility states

Tags admin:
- Database: getAllTagsWithCount(), renameTag(), mergeTag(), deleteTag()
- public/admin/tags.php: table with inline rename/merge/delete forms, CSRF-guarded
- public/admin/actions/tag.php: routes on action=rename|merge|delete
- templates/admin/head.php: 'Mots-clés' nav link
- admin.css: admin-inline-form, admin-btn--sm/warning/danger variants

Maintenance mode:
- config/bootstrap.php: gate on MAINTENANCE_FLAG file; admin/ and maintenance.php exempt
- public/maintenance.php: 503 dark minimal page
- public/admin/actions/maintenance.php: enable/disable toggle
- public/admin/index.php: status bar with toggle button
- admin.css: admin-maintenance-bar styles

TFE Visibility (Libre/Interne/Interdit via existing access_type_id):
- migration 002_add_visibility.sql: seeds access_types if missing
- Database: setVisibility(), bulkSetVisibility(), getAccessTypes()
- public/media.php: blocks thesis files for access_type_id=3
- public/tfe.php: shows access_type, context_note; hides file panel for Interdit
- public/admin/edit.php: access_type_id select + context_note textarea; saves both
- public/admin/index.php: three-state badge (Libre/Interne/Interdit) per row
- public/admin/actions/visibility.php: single + bulk visibility action handler
- admin.css: status-access badge variants

public/assets/admin.css

@@ -641,3 +641,96 @@ html, body {
   border-color: #e55;
   color: #e55;
 }
+
+/* Inline form actions (tags page) */
+.admin-inline-form {
+  display: flex;
+  align-items: center;
+  gap: .4rem;
+  flex-wrap: wrap;
+}
+
+.admin-input--inline {
+  padding: .28rem .5rem;
+  font-size: .82rem;
+}
+
+.admin-select--inline {
+  padding: .28rem .5rem;
+  font-size: .82rem;
+}
+
+.admin-btn--sm {
+  padding: .28rem .65rem;
+  font-size: .8rem;
+}
+
+.admin-btn--warning {
+  background: #7a5400;
+  border-color: #b87a00;
+}
+.admin-btn--warning:hover {
+  background: #9a6a00;
+}
+
+.admin-btn--danger {
+  background: #6b1a1a;
+  border-color: #a03030;
+}
+.admin-btn--danger:hover {
+  background: #8a2020;
+}
+
+/* Maintenance mode bar */
+.admin-maintenance-bar {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  background: #1a1a2e;
+  border: 1px solid #333;
+  border-radius: 4px;
+  padding: .65rem 1rem;
+  margin-bottom: 1.5rem;
+  font-size: .88rem;
+  color: var(--admin-text-muted);
+}
+
+.admin-maintenance-bar--active {
+  background: #2a1a00;
+  border-color: #7a5400;
+  color: #e0a030;
+}
+
+/* Visibility / access badges */
+.status-access {
+  display: inline-block;
+  font-size: .7rem;
+  padding: .1rem .4rem;
+  border-radius: 3px;
+  margin-top: .2rem;
+  background: #333;
+  color: #aaa;
+  letter-spacing: .03em;
+}
+
+.status-access--Libre,
+.status-access--libre {
+  background: #0a2a0a;
+  color: #6fbe6f;
+  border: 1px solid #2a5a2a;
+}
+
+.status-access--Interne,
+.status-access--interne {
+  background: #1a1a2a;
+  color: #7a8fcc;
+  border: 1px solid #3a4a8a;
+}
+
+.status-access--Interdit,
+.status-access--interdit {
+  background: #2a0a0a;
+  color: #cc6060;
+  border: 1px solid #7a2020;
+}