feat(notifications): grouped email digest, new music as a daily summary (#5346)

Nothing is emailed per event. New music (request_completed) goes out at
most once a day, at the summary hour in each user's own timezone, grouped
by artist. Everything else is batched: one email a window after the first
un-emailed item, holding whatever accumulated.

- Migration 0074: notification_email_settings (summary hour, batch window,
  admin-configurable) and user_notification_email_state (batch start, last
  sent, failures and retry_after per user and group). Existing rows are
  stamped emailed so the upgrade sends no backlog.
- The Notifier stamps emailed_at at write time when the recipient's email
  channel is off, so turning email on later doesn't send old items.
- Read rows are never selected. A row is stamped only after the mailer
  accepts, in one transaction with the state, against the read's clock, so
  a coalesced row updated mid-send stays pending.
- A failed send backs off 5m doubling to 6h; SMTP not configured just waits.
- Links come from the public address; without one the email has none.
- The mailer now RFC 2047-encodes subjects and strips line breaks from them.
- Admin → Integrations gains a Notification emails card.

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

web/src/lib/components/NotificationEmailCard.svelte

@@ -0,0 +1,126 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import {
+    getNotificationEmailSettings,
+    updateNotificationEmailSettings,
+    type NotificationEmailSettings
+  } from '$lib/api/admin';
+  import { errMessage } from '$lib/api/errors';
+  import { pushToast } from '$lib/stores/toast.svelte';
+
+  // When notification emails go out (M489 #5346). Nothing is emailed per
+  // event: new music is a daily summary, everything else a batch.
+
+  let saved = $state<NotificationEmailSettings | null>(null);
+  let form = $state<NotificationEmailSettings | null>(null);
+  let saving = $state(false);
+  let loadError = $state(false);
+
+  const dirty = $derived(
+    !!saved &&
+      !!form &&
+      (saved.summary_hour !== form.summary_hour ||
+        saved.batch_window_minutes !== form.batch_window_minutes)
+  );
+
+  const hours = Array.from({ length: 24 }, (_, h) => h);
+
+  function hourLabel(h: number): string {
+    return `${String(h).padStart(2, '0')}:00`;
+  }
+
+  async function load() {
+    try {
+      saved = await getNotificationEmailSettings();
+      form = { ...saved };
+      loadError = false;
+    } catch {
+      loadError = true;
+    }
+  }
+
+  onMount(load);
+
+  async function save() {
+    if (!form) return;
+    saving = true;
+    try {
+      saved = await updateNotificationEmailSettings(form);
+      form = { ...saved };
+      pushToast('Notification email schedule saved.');
+    } catch (e) {
+      // The server names the field it refused (#3937: errMessage, not
+      // e.message, because the client throws a plain object).
+      pushToast(errMessage(e), 'error');
+    } finally {
+      saving = false;
+    }
+  }
+</script>
+
+<section class="space-y-4 rounded-xl border border-border bg-surface p-5">
+  <div>
+    <h3 class="font-display text-lg font-medium text-text-primary">Notification emails</h3>
+    <p class="mt-1 text-sm text-text-secondary">
+      Nothing is emailed per event. Each person chooses which notifications reach their email in
+      their own settings.
+    </p>
+  </div>
+
+  {#if loadError}
+    <p class="text-sm text-action-destructive">
+      Couldn't load the email schedule.
+      <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 the email schedule…</p>
+  {:else}
+    <div class="grid gap-4 sm:grid-cols-2">
+      <label class="block">
+        <span class="text-sm text-text-primary">New music summary</span>
+        <span class="block text-xs text-text-secondary">
+          Once a day at this hour, in each person's own timezone.
+        </span>
+        <select
+          bind:value={form.summary_hour}
+          class="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"
+        >
+          {#each hours as h (h)}
+            <option value={h}>{hourLabel(h)}</option>
+          {/each}
+        </select>
+      </label>
+
+      <label class="block">
+        <span class="text-sm text-text-primary">Batch everything else (minutes)</span>
+        <span class="block text-xs text-text-secondary">
+          One email this long after the first item, holding all that came in.
+        </span>
+        <input
+          type="number"
+          min="15"
+          max="1440"
+          bind:value={form.batch_window_minutes}
+          class="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"
+        />
+      </label>
+    </div>
+
+    <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}
+        onclick={save}
+      >
+        {saving ? 'Saving…' : 'Save'}
+      </button>
+    </div>
+  {/if}
+</section>