docs(notifications): spec per-type notification preferences (email + bell)

[leahpeker]

Summary

• adds .claude/docs/spec-push-and-notification-prefs.md — phased spec for push notifications (web push first, Capacitor/FCM later) and per-type notification opt-outs
• key design points: opt-out-only NotificationOptOut model enforced once at notification creation (SSE + push inherit prefs for free), grouped settings toggles with a separate email toggle for types that also send email (event invites, co-host invites, check-in nudges), and splitting event_comment into event_comment_hosting / event_comment_attending — the attending fan-out is net-new (attendees currently get no comment notification)
• builds on spike-native-push.md and app-store-readiness.md; supersedes the spike's raw-APNs sender choice with FCM

Test plan

• docs-only change — review the spec content, especially the open questions section

[leahpeker]

rescoped: push cut, email + bell only

Confirmed there is no push infrastructure in the repo — no service worker, no Capacitor shell,
no FCM/APNs dependency. Push isn't wanted, so phases 3–5 are removed and the file is renamed
(spec-push-and-notification-prefs.md → spec-notification-prefs.md). The push research
survives in spike-native-push.md / app-store-readiness.md if it's ever revived.

Phases 1–2 stand: per-type opt-outs across the bell and email, then the hosting/attending
comment split.

Decisions folded in:

• checkin_nudge and rsvp_declined_note are host-facing and now always on — a host needs
  to know who can't make it and that check-in is open. Both dropped from the settings UI table.
• Bell and email are independently toggleable per type.
• Milestone-reminder and event-blast email keys stay deferred.
• Push-title question is moot and removed.

One open question left, which overlaps Issue 1143: RSVP confirmation emails are scoped to
send on all status transitions including waitlist promotion, so they need a NotificationType
to hang an email toggle on. waitlist_promoted exists (with no producer at all today), but a
plain "you're going" confirmation has no type yet. That gets decided here or in 1143.