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/cl.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-cl" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="cl-a">
+      <path fill-opacity=".7" d="M0 0h682.7v512H0z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#cl-a)" transform="scale(.9375)">
+    <path fill="#fff" d="M256 0h512v256H256z"/>
+    <path fill="#0039a6" d="M0 0h256v256H0z"/>
+    <path fill="#fff" d="M167.8 191.7 128.2 162l-39.5 30 14.7-48.8L64 113.1l48.7-.5L127.8 64l15.5 48.5 48.7.1-39.2 30.4z"/>
+    <path fill="#d52b1e" d="M0 256h768v256H0z"/>
+  </g>
+</svg>