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/tz.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-tz" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="tz-a">
+      <path fill-opacity=".7" d="M10 0h160v120H10z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#tz-a)" transform="matrix(4 0 0 4 -40 0)">
+    <path fill="#09f" d="M0 0h180v120H0z"/>
+    <path fill="#090" d="M0 0h180L0 120z"/>
+    <path fill="#000001" d="M0 120h40l140-95V0h-40L0 95z"/>
+    <path fill="#ff0" d="M0 91.5 137.2 0h13.5L0 100.5zM29.3 120 180 19.5v9L42.8 120z"/>
+  </g>
+</svg>