feat(library): measure every track's loudness in the background (M464 #4995)

The first step of loudness normalization: the server measures each track
with ffmpeg's EBU R128 filter (true peak, mono as dual mono) and stores the
integrated loudness, true peak and loudness range in track_loudness
(migration 0065).

It also keeps a histogram of the 400 ms gating blocks at 0.1 LU, so album
loudness can be computed exactly later with no second decode (#4996). The
histogram reproduces ffmpeg's own figure (-10.68 against -10.7 on the
captured fixture), and the analyzer logs a warning if the two ever drift.

- A background worker, cloned from the fingerprint backfill, measures every
  track, new ones included. Measuring inline in the scan was dropped: the
  analysis decodes the whole file, and a large import could pass the scan's
  one-hour stuck threshold. The scan only deletes a changed file's
  measurement; the worker ticks every 10 minutes.
- Timeouts, the cancel/missing-binary split and settled verdicts follow the
  fingerprint runner. Silence and undecodable files are stored as verdicts;
  stalls are retried. The deadline scales with track length.
- loudness_settings (enabled, files at once) and an admin card with the
  coverage gauge, under GET/PUT /api/admin/library/loudness-settings and
  GET /api/admin/library/loudness.

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

web/src/lib/components/LoudnessSettingsCard.svelte

@@ -0,0 +1,173 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import {
+    getLoudnessCoverage,
+    getLoudnessSettings,
+    updateLoudnessSettings,
+    type LoudnessCoverage,
+    type LoudnessSettings
+  } from '$lib/api/admin';
+  import { errMessage } from '$lib/api/errors';
+  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.
+
+  let saved = $state<LoudnessSettings | null>(null);
+  let form = $state<LoudnessSettings | null>(null);
+  let coverage = $state<LoudnessCoverage | null>(null);
+  let saving = $state(false);
+  let loadError = $state(false);
+
+  const dirty = $derived(!!saved && !!form && JSON.stringify(saved) !== JSON.stringify(form));
+  const concurrencyOk = $derived(
+    !!form &&
+      Number.isInteger(form.backfill_concurrency) &&
+      form.backfill_concurrency >= 1 &&
+      form.backfill_concurrency <= 8
+  );
+
+  async function load() {
+    try {
+      saved = await getLoudnessSettings();
+      form = { ...saved };
+      loadError = false;
+    } catch {
+      loadError = true;
+    }
+    await refreshCoverage();
+  }
+
+  // The gauge moves a few tracks a minute while the backfill runs, so a slow
+  // poll is enough; a failed read keeps the last figures rather than blanking.
+  async function refreshCoverage() {
+    try {
+      coverage = await getLoudnessCoverage();
+    } catch {
+      // Keep what we had.
+    }
+  }
+
+  onMount(() => {
+    load();
+    const timer = setInterval(refreshCoverage, 30_000);
+    return () => clearInterval(timer);
+  });
+
+  async function save() {
+    if (!form || !concurrencyOk) return;
+    saving = true;
+    try {
+      saved = await updateLoudnessSettings(form);
+      form = { ...saved };
+      pushToast('Loudness analysis settings saved.');
+      await refreshCoverage();
+    } catch (e) {
+      pushToast(errMessage(e), 'error');
+    } finally {
+      saving = false;
+    }
+  }
+
+  const inputClass =
+    'mt-1 w-28 rounded border border-border bg-background px-2 py-1 text-sm text-text-primary ' +
+    'focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent';
+</script>
+
+<section class="space-y-4 rounded border border-border bg-surface p-4">
+  <div>
+    <h2 class="font-display text-lg font-medium">Loudness analysis</h2>
+    <p class="mt-1 text-sm text-text-secondary">
+      Measures how loud each track is, so playback can even out the volume between tracks. It runs
+      in the background; until a track is measured, it plays at its own volume.
+    </p>
+  </div>
+
+  {#if coverage && coverage.total > 0}
+    <div class="flex flex-wrap items-center gap-3 text-sm" data-testid="loudness-coverage">
+      <span>
+        {coverage.measured.toLocaleString()} of {coverage.total.toLocaleString()} tracks measured
+      </span>
+      {#if coverage.pending > 0}
+        <span class="text-text-muted">·</span>
+        <span>{coverage.pending.toLocaleString()} pending</span>
+      {/if}
+      {#if !coverage.enabled && coverage.pending > 0}
+        <span class="text-text-muted">·</span>
+        <span>paused while analysis is off</span>
+      {/if}
+      {#if coverage.silent > 0}
+        <span class="text-text-muted">·</span>
+        <span
+          class="cursor-help"
+          title="These tracks are silent or too short to measure. They play without adjustment."
+        >
+          {coverage.silent.toLocaleString()} silent
+        </span>
+      {/if}
+      {#if coverage.unreadable > 0}
+        <span class="text-text-muted">·</span>
+        <span
+          class="cursor-help"
+          title="ffmpeg couldn't decode these files. Each is tried again when its file changes."
+        >
+          {coverage.unreadable.toLocaleString()} unreadable
+        </span>
+      {/if}
+    </div>
+  {/if}
+
+  {#if loadError}
+    <p class="text-sm text-action-destructive">
+      Couldn't load loudness analysis settings.
+      <button type="button" class="underline hover:no-underline" onclick={load}>Try again</button>
+    </p>
+  {:else if form === null}
+    <p class="text-sm text-text-secondary">Loading…</p>
+  {:else}
+    <label class="flex items-start gap-3">
+      <input type="checkbox" bind:checked={form.enabled} class="mt-1" />
+      <span>
+        <span class="text-sm text-text-primary">Analyze tracks</span>
+        <span class="block text-xs text-text-secondary">
+          Off stops the background analysis, including for new tracks. Tracks already measured keep
+          their values, so they stay leveled.
+        </span>
+      </span>
+    </label>
+
+    <label class="block">
+      <span class="text-sm text-text-primary">Files analyzed at once</span>
+      <span class="block text-xs text-text-secondary">
+        Each analysis reads the whole file, competing with playback for CPU and with streaming for
+        the disk. Lower it if playback stutters while it runs.
+      </span>
+      <input
+        type="number"
+        min="1"
+        max="8"
+        bind:value={form.backfill_concurrency}
+        class={inputClass}
+      />
+    </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>
+    {/if}
+
+    <div class="flex justify-end">
+      <button
+        type="button"
+        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}
+        onclick={save}
+      >
+        {saving ? 'Saving…' : 'Save'}
+      </button>
+    </div>
+  {/if}
+</section>