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/cu.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-cu" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="cu-a">
+      <path fill-opacity=".7" d="M-32 0h682.7v512H-32z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#cu-a)" transform="translate(30)scale(.94)">
+    <path fill="#002a8f" d="M-32 0h768v512H-32z"/>
+    <path fill="#fff" d="M-32 102.4h768v102.4H-32zm0 204.8h768v102.4H-32z"/>
+    <path fill="#cb1515" d="m-32 0 440.7 255.7L-32 511z"/>
+    <path fill="#fff" d="M161.8 325.5 114.3 290l-47.2 35.8 17.6-58.1-47.2-36 58.3-.4 18.1-58 18.5 57.8 58.3.1-46.9 36.3z"/>
+  </g>
+</svg>