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>

internal/mailer/subject_test.go

@@ -0,0 +1,25 @@
+package mailer
+
+import (
+	"mime"
+	"strings"
+	"testing"
+)
+
+func TestEncodeSubject(t *testing.T) {
+	if got := encodeSubject("Minstrel: 2 updates"); got != "Minstrel: 2 updates" {
+		t.Errorf("plain ASCII changed: %q", got)
+	}
+
+	enc := encodeSubject("Minstrel: Moe Shop – WWW")
+	if !strings.HasPrefix(enc, "=?utf-8?q?") {
+		t.Errorf("non-ASCII not RFC 2047 encoded: %q", enc)
+	}
+	if dec, err := new(mime.WordDecoder).DecodeHeader(enc); err != nil || dec != "Minstrel: Moe Shop – WWW" {
+		t.Errorf("round trip = %q, %v", dec, err)
+	}
+
+	if got := encodeSubject("Hi\r\nBcc: everyone@example.com"); strings.ContainsAny(got, "\r\n") {
+		t.Errorf("a line break survived into the header: %q", got)
+	}
+}