feat(player): let editors upload subtitles for a version

Subtitle tracks hang off a version rather than off a video, because re-editing a cut shifts every cue. The file
always lands in our own S3-compatible storage whatever hosts the video, so a Bunny-hosted cut and an R2 one take the
same path: both already play through our own video element, so a track element is all it takes.

Uploads are normalised before they are stored. Whatever arrives, SRT or WebVTT, is parsed into cues and
re-serialised as a canonical WebVTT file, and anything we did not understand is dropped rather than passed through.
That is what makes it safe to serve a user-supplied text file from our own origin. Files saved out of Windows
editors are decoded as windows-1254 or windows-1252 when they are not valid UTF-8, rather than refused.

A YouTube version cannot carry an uploaded track, so the same CC menu drives YouTube's own captions through the
iframe module API. The embed hides YouTube's controls, so until now those captions were unreachable even when the
video had them.

Uploading and deleting take the editor permission rather than the commenter one: a subtitle is part of the
delivered cut, not a comment attachment.

components/video-page/hooks/use-video-player.ts

@@ -81,6 +81,10 @@ export function useVideoPlayer({
 }: UseVideoPlayerParams) {
   const [isApiLoaded, setIsApiLoaded] = useState(false);
   const [isReady, setIsReady] = useState(false);
+  // Bumped every time the YouTube player loads or unloads a module. It is the only
+  // signal that `getOption('captions', ...)` will answer, so the captions hook waits
+  // on it rather than polling.
+  const [youtubeModuleRevision, setYoutubeModuleRevision] = useState(0);
   const [bunnyPlaybackState, setBunnyPlaybackState] = useState<BunnyPlaybackState>('none');
   const [currentTime, setCurrentTime] = useState(0);
   const [videoDuration, setVideoDuration] = useState(0);
@@ -315,6 +319,9 @@ export function useVideoPlayer({
               const dur = event.target.getDuration();
               if (dur > 0) setVideoDuration(dur);
             },
+            onApiChange: () => {
+              setYoutubeModuleRevision((revision) => revision + 1);
+            },
             onStateChange: (event: YT.OnStateChangeEvent) => {
               setIsPlaying(event.data === YT.PlayerState.PLAYING);
 
@@ -1344,6 +1351,7 @@ export function useVideoPlayer({
 
   return {
     isReady,
+    youtubeModuleRevision,
     bunnyPlaybackState,
     currentTime,
     setCurrentTime,