feat: leveled FLAC stream for Sonos/UPnP speakers (M464 #5001)

Speakers fetch their own audio, so the phone cannot level it. A cast
token minted with level=true (and the client's asAlbum, which only the
queue holder knows) now returns GET /api/tracks/{id}/leveled.flac: the
track rendered by ffmpeg at the user's gain (volume=XdB, plus
alimiter at -1 dBFS for a limiter-mode boost), metadata stripped, FLAC
at 16 or 24 bits and at most 48 kHz. The gain is computed server-side
from the user's preference and the stored loudness, carried as
?g=<centi-dB>&lim=0|1 and signed into the token, so an edited URL does
not verify. Unity gains get the plain stream.

Renders are written beside the cache file and renamed in, keyed by the
source's size and mtime, coalesced per file (singleflight, detached
from the requesting speaker so a retry finds the render running),
started at mint time so the fetch finds them ready, and evicted least
recently used past leveled_cache_mb, a new admin setting (migration
0068, Loudness analysis card).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/api/admin.ts

@@ -386,6 +386,8 @@ export async function getLoudnessCoverage(): Promise<LoudnessCoverage> {
 export type LoudnessSettings = {
   enabled: boolean;
   backfill_concurrency: number;
+  /** Disk for the leveled copies rendered for Sonos/UPnP speakers (#5001). */
+  leveled_cache_mb: number;
 };
 
 export async function getLoudnessSettings(): Promise<LoudnessSettings> {

web/src/lib/components/LoudnessSettingsCard.svelte

@@ -11,7 +11,8 @@
   import { pushToast } from '$lib/stores/toast.svelte';
 
   // Loudness analysis (#4995): the background measurement that loudness
-  // normalization levels playback from, its progress, and its two knobs.
+  // normalization levels playback from, its progress, and its knobs. The
+  // cache size bounds the leveled copies rendered for speakers (#5001).
 
   let saved = $state<LoudnessSettings | null>(null);
   let form = $state<LoudnessSettings | null>(null);
@@ -26,6 +27,13 @@
       form.backfill_concurrency >= 1 &&
       form.backfill_concurrency <= 8
   );
+  const cacheOk = $derived(
+    !!form &&
+      Number.isInteger(form.leveled_cache_mb) &&
+      form.leveled_cache_mb >= 256 &&
+      form.leveled_cache_mb <= 65536
+  );
+  const valid = $derived(concurrencyOk && cacheOk);
 
   async function load() {
     try {
@@ -55,7 +63,7 @@
   });
 
   async function save() {
-    if (!form || !concurrencyOk) return;
+    if (!form || !valid) return;
     saving = true;
     try {
       saved = await updateLoudnessSettings(form);
@@ -151,10 +159,26 @@
       />
     </label>
 
-    {#if !concurrencyOk}
-      <p class="text-xs text-action-destructive" data-testid="settings-problems">
-        Files analyzed at once must be a whole number from 1 to 8.
-      </p>
+    <label class="block">
+      <span class="text-sm text-text-primary">Speaker cache (MB)</span>
+      <span class="block text-xs text-text-secondary">
+        Leveled copies for Sonos and UPnP speakers. The least recently played go first.
+      </span>
+      <input
+        type="number"
+        min="256"
+        max="65536"
+        step="256"
+        bind:value={form.leveled_cache_mb}
+        class={inputClass}
+      />
+    </label>
+
+    {#if !valid}
+      <div class="text-xs text-action-destructive" data-testid="settings-problems">
+        {#if !concurrencyOk}<p>Files analyzed at once must be a whole number from 1 to 8.</p>{/if}
+        {#if !cacheOk}<p>Speaker cache must be a whole number of MB from 256 to 65536.</p>{/if}
+      </div>
     {/if}
 
     <div class="flex justify-end">
@@ -163,7 +187,7 @@
         class="rounded-md bg-action-secondary px-4 py-2 text-sm text-action-fg hover:opacity-90
                focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent
                disabled:cursor-not-allowed disabled:opacity-50"
-        disabled={!dirty || saving || !concurrencyOk}
+        disabled={!dirty || saving || !valid}
         onclick={save}
       >
         {saving ? 'Saving…' : 'Save'}

web/src/lib/components/LoudnessSettingsCard.test.ts

@@ -14,7 +14,7 @@ import LoudnessSettingsCard from './LoudnessSettingsCard.svelte';
 import { getLoudnessCoverage, getLoudnessSettings, updateLoudnessSettings } from '$lib/api/admin';
 import { pushToast } from '$lib/stores/toast.svelte';
 
-const base: LoudnessSettings = { enabled: true, backfill_concurrency: 2 };
+const base: LoudnessSettings = { enabled: true, backfill_concurrency: 2, leveled_cache_mb: 2048 };
 const coverage: LoudnessCoverage = {
   total: 1200,
   measured: 900,
@@ -68,7 +68,11 @@ describe('LoudnessSettingsCard', () => {
     await fireEvent.click(saveButton());
 
     await waitFor(() =>
-      expect(updateLoudnessSettings).toHaveBeenCalledWith({ enabled: true, backfill_concurrency: 4 })
+      expect(updateLoudnessSettings).toHaveBeenCalledWith({
+        enabled: true,
+        backfill_concurrency: 4,
+        leveled_cache_mb: 2048
+      })
     );
     await waitFor(() => expect(pushToast).toHaveBeenCalledWith('Loudness analysis settings saved.'));
     // The gauge is read again after a save: switching analysis on or off changes it.
@@ -84,6 +88,16 @@ describe('LoudnessSettingsCard', () => {
     expect(saveButton()).toHaveProperty('disabled', true);
   });
 
+  test('the speaker cache size is bounded too', async () => {
+    await renderCard();
+    const cache = screen.getByRole('spinbutton', { name: /speaker cache/i });
+    await fireEvent.input(cache, { target: { value: '100' } });
+    await waitFor(() =>
+      expect(screen.getByTestId('settings-problems').textContent).toMatch(/256 to 65536/)
+    );
+    expect(saveButton()).toHaveProperty('disabled', true);
+  });
+
   test('a failed load offers a retry', async () => {
     vi.mocked(getLoudnessSettings).mockRejectedValue(new Error('boom'));
     vi.mocked(getLoudnessCoverage).mockResolvedValue(coverage);

web/src/routes/admin/admin.test.ts

@@ -50,7 +50,9 @@ vi.mock('$lib/api/admin', async () => {
     refetchMissingCovers: vi.fn().mockResolvedValue({ started: true }),
     researchMissingArt: vi.fn().mockResolvedValue({ version: 1 }),
     // LoudnessSettingsCard, rendered by the page and tested on its own.
-    getLoudnessSettings: vi.fn().mockResolvedValue({ enabled: true, backfill_concurrency: 2 }),
+    getLoudnessSettings: vi
+      .fn()
+      .mockResolvedValue({ enabled: true, backfill_concurrency: 2, leveled_cache_mb: 2048 }),
     updateLoudnessSettings: vi.fn(),
     getLoudnessCoverage: vi.fn().mockResolvedValue({
       total: 0, measured: 0, silent: 0, unreadable: 0, pending: 0, enabled: true