feat(notifications): the inbox store, one writer, coalescing and retention (#5338)

M489 step 1. The event bus is fire-and-forget, so a client that isn't
connected never hears that a request completed or that tracks went missing.
user_notifications is the durable record; the bus only nudges.

- Migration 0073: user_notifications (kind CHECK-gated, payload jsonb,
  read_at, coalesce_key, emailed_at) and user_notification_prefs (per user,
  per kind: inbox, phone, email). A missing pref row means the kind's
  defaults, so nothing is seeded.
- internal/notifications.Notifier is the only writer. It resolves recipients
  (admin kinds reach admins only, and never the excepted user), honours the
  inbox pref (phone and email ride on it), writes, and publishes a
  contentless notification.created nudge per recipient.
- Burst-prone admin kinds coalesce into one unread row: tracks_missing and
  scan_failed add up their counts, duplicates_found and playback_errors take
  the latest total. Once read, the next event is a new row.
- Retention: read rows go after 90 days, anything after a year, on the
  library_changes compactor's daily shape.

Tests: unit (channel rules, kind table) and integration (recipients, nudge,
coalescing both ways, prefs, owner-scoped idempotent mark-read, every kind
against both schema CHECKs, retention cut-offs).

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

internal/db/dbq/models.go

@@ -831,6 +831,26 @@ type UserNormalizationPref struct {
 	UpdatedAt  pgtype.Timestamptz
 }
 
+type UserNotification struct {
+	ID          pgtype.UUID
+	UserID      pgtype.UUID
+	Kind        string
+	Payload     []byte
+	CreatedAt   pgtype.Timestamptz
+	ReadAt      pgtype.Timestamptz
+	CoalesceKey *string
+	EmailedAt   pgtype.Timestamptz
+}
+
+type UserNotificationPref struct {
+	UserID    pgtype.UUID
+	Kind      string
+	Inbox     bool
+	Phone     bool
+	Email     bool
+	UpdatedAt pgtype.Timestamptz
+}
+
 type YouMightLikeAlbum struct {
 	UserID  pgtype.UUID
 	AlbumID pgtype.UUID