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/dj.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-dj" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="dj-a">
+      <path fill-opacity=".7" d="M-40 0h682.7v512H-40z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#dj-a)" transform="translate(37.5)scale(.94)">
+    <path fill="#0c0" d="M-40 0h768v512H-40z"/>
+    <path fill="#69f" d="M-40 0h768v256H-40z"/>
+    <path fill="#fffefe" d="m-40 0 382.7 255.7L-40 511z"/>
+    <path fill="red" d="M119.8 292 89 270l-30.7 22.4L69.7 256l-30.6-22.5 37.9-.3 11.7-36.3 12 36.2h37.9l-30.5 22.7 11.7 36.4z"/>
+  </g>
+</svg>