add tagging system for media items

Introduces user-defined tag categories and tags with a many-to-many
relationship to media items. Tags are stored in a SQLite database
(medialore.db via better-sqlite3) with ON DELETE CASCADE for automatic
cleanup. Users can manage categories and tags at /manage/tags, assign
tags to games in the detail modal, and tag mixed media files via a
hover button on each tile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/manage/layout.tsx

@@ -0,0 +1,10 @@
+import ManageSubNav from '@/components/ManageSubNav'
+
+export default function ManageLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <div>
+      <ManageSubNav />
+      {children}
+    </div>
+  )
+}