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.

prisma/migrations/20260822120000_add_video_subtitles/migration.sql

@@ -0,0 +1,39 @@
+-- Subtitle tracks hang off a version, not off the video: re-editing a cut shifts
+-- every cue, so a track attached to the parent would be wrong for every version
+-- but the one it was written against.
+CREATE TABLE "video_subtitles" (
+    "id" TEXT NOT NULL,
+    "versionId" TEXT NOT NULL,
+    "language" TEXT NOT NULL,
+    "label" TEXT NOT NULL,
+    "sourceUrl" TEXT NOT NULL,
+    "size_bytes" BIGINT NOT NULL DEFAULT 0,
+    "billedUserId" TEXT NOT NULL,
+    "uploadedByUserId" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "video_subtitles_pkey" PRIMARY KEY ("id")
+);
+
+-- One stored object belongs to exactly one row, so the reference check that runs
+-- before an object delete cannot be fooled by a second row pointing at the file.
+CREATE UNIQUE INDEX "video_subtitles_sourceUrl_key" ON "video_subtitles"("sourceUrl");
+
+-- Re-uploading a language replaces the track rather than stacking a second one,
+-- which would leave the player with two tracks labelled the same.
+CREATE UNIQUE INDEX "video_subtitles_versionId_language_key" ON "video_subtitles"("versionId", "language");
+
+CREATE INDEX "video_subtitles_versionId_idx" ON "video_subtitles"("versionId");
+
+-- The storage quota sums this column per billed user on every upload.
+CREATE INDEX "video_subtitles_billedUserId_idx" ON "video_subtitles"("billedUserId");
+
+ALTER TABLE "video_subtitles" ADD CONSTRAINT "video_subtitles_versionId_fkey"
+    FOREIGN KEY ("versionId") REFERENCES "video_versions"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+ALTER TABLE "video_subtitles" ADD CONSTRAINT "video_subtitles_billedUserId_fkey"
+    FOREIGN KEY ("billedUserId") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+ALTER TABLE "video_subtitles" ADD CONSTRAINT "video_subtitles_uploadedByUserId_fkey"
+    FOREIGN KEY ("uploadedByUserId") REFERENCES "users"("id") ON DELETE SET NULL ON UPDATE CASCADE;

prisma/schema.prisma

@@ -42,6 +42,8 @@ model User {
   comments           Comment[]
   uploadedVideoAssets VideoAsset[] @relation("VideoAssetUploadedBy")
   billedVideoAssets   VideoAsset[] @relation("VideoAssetBilledTo")
+  uploadedVideoSubtitles VideoSubtitle[] @relation("VideoSubtitleUploadedBy")
+  billedVideoSubtitles   VideoSubtitle[] @relation("VideoSubtitleBilledTo")
   projectMemberships ProjectMember[]
   notificationSetting NotificationSetting?
   watchProgress      WatchProgress[]
@@ -383,6 +385,7 @@ model VideoVersion {
   comments        Comment[]
   watchProgress   WatchProgress[]
   approvalRequests ApprovalRequest[]
+  subtitles       VideoSubtitle[]
   
   @@unique([videoParentId, versionNumber])
   @@index([videoParentId])
@@ -418,6 +421,34 @@ model VideoAsset {
   @@map("video_assets")
 }
 
+/// A subtitle track for one cut. Timings belong to a version rather than to the
+/// video: re-editing shifts every cue, so a track attached to the parent would be
+/// wrong for every version but the one it was written against.
+model VideoSubtitle {
+  id               String       @id @default(cuid())
+  versionId        String
+  version          VideoVersion @relation(fields: [versionId], references: [id], onDelete: Cascade)
+  /// BCP-47 tag, lowercased primary subtag, e.g. `tr`, `en-US`.
+  language         String
+  label            String
+  /// Always an /api/upload/subtitle/<uuid>.vtt path. The file itself lives in
+  /// S3-compatible storage whatever the video's own provider is, so a Bunny-hosted
+  /// video and an R2-hosted one take the same path through the player.
+  sourceUrl        String       @unique
+  sizeBytes        BigInt       @default(0) @map("size_bytes")
+  billedUserId     String
+  billedUser       User         @relation("VideoSubtitleBilledTo", fields: [billedUserId], references: [id], onDelete: Cascade)
+  uploadedByUserId String?
+  uploadedByUser   User?        @relation("VideoSubtitleUploadedBy", fields: [uploadedByUserId], references: [id], onDelete: SetNull)
+  createdAt        DateTime     @default(now())
+  updatedAt        DateTime     @updatedAt
+
+  @@unique([versionId, language])
+  @@index([versionId])
+  @@index([billedUserId])
+  @@map("video_subtitles")
+}
+
 model Comment {
   id            String      @id @default(cuid())