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/notifications/render.go

@@ -24,7 +24,12 @@ type Payload struct {
 	RequestKind string `json:"request_kind,omitempty"` // artist | album | track
 	// Name is what was requested or flagged, as the user would say it:
 	// "WWW", "Moe Shop", "Moe Shop – WWW".
-	Name     string `json:"name,omitempty"`
+	Name string `json:"name,omitempty"`
+	// Artist and Title are Name's parts, for request_completed: the new-music
+	// summary email groups arrivals by artist. Title is empty for an artist
+	// request.
+	Artist   string `json:"artist,omitempty"`
+	Title    string `json:"title,omitempty"`
 	ArtistID string `json:"artist_id,omitempty"`
 	AlbumID  string `json:"album_id,omitempty"`
 	// Actor is the other person involved: who asked, who flagged.