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/nr.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-nr" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="nr-a">
+      <path fill-opacity=".7" d="M-54.7 0H628v512H-54.7z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#nr-a)" transform="translate(51.3)scale(.9375)">
+    <path fill="#002170" d="M-140 0H884v512H-140z"/>
+    <path fill="#ffb20d" d="M-140 234.1H884V278H-140z"/>
+    <path fill="#fff" d="m161.8 438-33-33-10.5 45.4-12-45-31.9 34 12.1-45L42 407.9l33-33-45.4-10.6 45-12-34-31.8 45 12L72 288l33 33 10.6-45.4 12 45 31.8-34-12 45 44.5-13.5-33 33 45.4 10.5-45 12 34 32-45-12.2z"/>
+  </g>
+</svg>