Add a subtitle picker to the video player

The API could serve tracks but nothing asked for them. The picker lists what
the item has and attaches one <track> at a time, removing the previous element
first so switching never accumulates them.

Default is Off. Auto-selecting a track would turn subtitles on for people who
did not ask and cannot tell why, and the failure mode is worse than a missing
feature: a track that loads over a film you did not want subtitled.

Subtitles never block playback. A failed track list hides the picker, and a
track that fails to load leaves the video playing - the media element's own
state is never touched by any of this.

Not verified by a test: the web package has no test runner, only svelte-check,
so verification here is the type and a11y check (0 errors, 0 warnings) plus
the API end-to-end below.

Verified end-to-end against a running server: a video with "Show.en.srt" and
"Show.pt-BR.srt" beside it produced two sidecar rows with the right languages,
the list route returned both, and the track route served real WebVTT - header,
Content-Type text/vtt, and the SRT commas converted to dots.

web/src/lib/api.ts

@@ -214,6 +214,10 @@ export const api = {
 
   fileUrlById: (id: number, fileId: number) => `/api/v1/items/${id}/files/${fileId}`,
 
+  subtitles: (id: number) => request<{ subtitles: Subtitle[] }>('GET', `/api/v1/items/${id}/subtitles`),
+
+  subtitleUrl: (id: number, trackId: number) => `/api/v1/items/${id}/subtitles/${trackId}`,
+
   progress: (id: number) => request<{ progress: Progress | null }>('GET', `/api/v1/items/${id}/progress`),
 
   putProgress: (
@@ -316,6 +320,21 @@ export interface ItemDetail {
   progress?: Progress
 }
 
+// Subtitle carries the facts about one subtitle track. The server renders every
+// format as WebVTT on the way out, so a client only needs the id to build the
+// track URL; Path and StreamIndex stay server-side.
+export interface Subtitle {
+  id: number
+  /** Where the track came from: "embedded", "sidecar" or "downloaded". */
+  source: string
+  /** Language tag, or "" when the file did not carry one. */
+  language: string
+  /** Human label for the picker, e.g. "English (SDH)". */
+  label: string
+  /** Original format, e.g. "srt", "ass", "vtt". */
+  format: string
+}
+
 // ── playback engines ────────────────────────────────────────────────────
 // Playability is a property of the CLIENT, so a client declares the engines it
 // has and the server matches them against the file's codec facts. A browser

web/src/lib/components/VideoPlayer.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { onMount, onDestroy } from 'svelte'
-  import { api, ApiError, type Item, type MediaFile } from '../api'
+  import { api, ApiError, type Item, type MediaFile, type Subtitle } from '../api'
   import * as playback from '../playback'
   import { clientEngines, probeFile } from '../capabilities'
   import {
@@ -42,6 +42,12 @@
   let currentTime = $state(0)
   let duration = $state(0)
 
+  // Subtitle tracks for the item, and the one the viewer picked. Empty list =
+  // no picker (the item has none, or the list could not be loaded); null =
+  // Off, which is always the default — a track is never auto-selected.
+  let subtitles = $state<Subtitle[]>([])
+  let subtitleId = $state<number | null>(null)
+
   // ── Honest-path state ────────────────────────────────────────────────
   // `notice` is non-empty only when this client genuinely cannot decode the
   // video track; `undecodable` drives the "audio only" presentation. Both are
@@ -110,6 +116,42 @@
     }
   }
 
+  // ── Subtitles ────────────────────────────────────────────────────────
+  // One <track> is mirrored onto the element from `subtitleId`. The old one is
+  // removed first, so switching never accumulates tracks. Best-effort by
+  // design: a track that fails to load is the browser's problem and leaves
+  // playback running, and an empty list simply shows no picker.
+  $effect(() => {
+    const el = video
+    const id = subtitleId
+    el?.querySelectorAll('track').forEach((t) => t.remove())
+    if (!el || id === null) return
+    const track = subtitles.find((s) => s.id === id)
+    if (!track) return
+    const node = document.createElement('track')
+    node.kind = 'subtitles'
+    node.src = api.subtitleUrl(itemId, track.id)
+    node.label = track.label
+    node.srclang = track.language || 'und'
+    node.default = true
+    el.appendChild(node)
+  })
+
+  // The list is fetched when the player opens. A failure is never surfaced:
+  // without a list there is no picker, but the video is unaffected.
+  async function loadSubtitles() {
+    try {
+      subtitles = (await api.subtitles(itemId)).subtitles ?? []
+    } catch {
+      subtitles = []
+    }
+  }
+
+  function selectSubtitle(event: Event) {
+    const value = (event.currentTarget as HTMLSelectElement).value
+    subtitleId = value === 'off' ? null : Number(value)
+  }
+
   onMount(async () => {
     // Persist the final position even if the page is torn down: keepalive lets a
     // PUT outlive the document, which sendBeacon (POST-only) cannot. Progress is
@@ -139,6 +181,7 @@
       // file. Best-effort: the element watchdog below is still the authority,
       // so this never blocks the player and a failure is never shown as an error.
       void refinePlayback()
+      void loadSubtitles()
     } catch (err) {
       error = err instanceof ApiError ? err.message : 'could not open this video'
     } finally {
@@ -550,10 +593,11 @@
 
     <div class="stage">
       <!-- svelte-ignore a11y_media_has_caption -->
-      <!-- Direct play only: Vellum serves the file untouched and has no subtitle
+      <!-- Direct play only: Vellum serves the file untouched. The subtitle
-           tracks to offer, so there is nothing to caption. src is on the element
+           <track> is added imperatively (see the subtitles $effect), so the
-           (not a <source>) so the browser actually loads the file and reports
+           a11y caption check cannot see it; the ignore stays. src is on the
-           videoWidth / error; see videoAttrs above. -->
+           element (not a <source>) so the browser actually loads the file and
+           reports videoWidth / error; see videoAttrs above. -->
       <video
         bind:this={video}
         class="video"
@@ -604,6 +648,22 @@
       <button class="btn btn-ghost" onclick={toggleFullscreen} title="Fullscreen (F)">Fullscreen</button>
     </div>
 
+    {#if subtitles.length > 0}
+      <div class="subs">
+        <label class="muted" for="subtitle-track">Subtitles</label>
+        <select
+          id="subtitle-track"
+          value={subtitleId === null ? 'off' : String(subtitleId)}
+          onchange={selectSubtitle}
+        >
+          <option value="off">Off</option>
+          {#each subtitles as s (s.id)}
+            <option value={String(s.id)}>{s.label || s.language || 'Subtitle'}</option>
+          {/each}
+        </select>
+      </div>
+    {/if}
+
     <p class="hint muted">Space play/pause · ←/→ seek {SEEK_STEP}s · F fullscreen · M mute</p>
 
     {#if files.length > 1}
@@ -729,6 +789,16 @@
     font-size: var(--fs-xs);
     text-align: center;
   }
+  .subs {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: var(--space-2);
+    font-size: var(--fs-sm);
+  }
+  .subs select {
+    padding: 4px var(--space-2);
+  }
   .files {
     list-style: none;
     margin: var(--space-2) 0 0;