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/bg.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-bg" viewBox="0 0 640 480">
+  <path fill="#fff" d="M0 0h640v160H0z"/>
+  <path fill="#00966e" d="M0 160h640v160H0z"/>
+  <path fill="#d62612" d="M0 320h640v160H0z"/>
+</svg>