Add multi-language audio tracks and self-hosted flag-icons

Introduce per-video language support and multiple audio tracks
(VideoAudioTrack model + migrations for language, description, title),
a reusable language-select component, and a track-editor form. Bundle
the self-hosted flag-icons v7.2.3 library and a NAS auto-sync command.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

public/vendor/flag-icons/flags/4x3/ch.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ch" viewBox="0 0 640 480">
+  <g fill-rule="evenodd" stroke-width="1pt">
+    <path fill="red" d="M0 0h640v480H0z"/>
+    <g fill="#fff">
+      <path d="M170 195h300v90H170z"/>
+      <path d="M275 90h90v300h-90z"/>
+    </g>
+  </g>
+</svg>