Record link copies, failed AI drafts and refused publishes #500

Needs your OK: this adds activity events and changes docs/telemetry.md. Swap pipeline:proposed for pipeline:ready to have it built. Part of #498. Ship it before the review page, so that page's effect can be measured.

What happens

The activity log can't see several of the steps where new organizers stop:

• Copying or sharing the public link records nothing. That applies to PublicLinkChip and the published banner. So "published but never shared" can't be measured.
• Magic Compose failures and refusals only reach the server log. "Tried the AI, it failed, gave up" is invisible.
• A refused publish leaves no trace.
• Organizer events carry no device class. Drafts abandoned on a phone can't be told apart from drafts abandoned on a computer.
• signup.draft_started doesn't say which form was shown, Magic Compose or the manual one. It fires on every render of the new-signup page.

Fix, in outline

• signup.link_copied and signup.link_shared (the second is for the phone share sheet). Send them from the browser on the landing.cta_clicked pattern, but behind the organizer session rather than a per-IP rate limit.
• magic_compose.failed, carrying the closed error code (not_configured | rate_limited | upstream | invalid_json | schema_mismatch | timeout | aborted, or refusal). Write it outside a transaction, as other telemetry-only events are.
• signup.publish_refused, carrying the error code.
• uaClass on organizer page-view events, as public views already have.
• mode: 'magic' | 'manual' on signup.draft_started.
• Reserve two events for the review-page checks: review.notice_shown and review.notice_accepted, carrying only the kind of notice.

Watch for

• Payloads hold codes and kinds only. No emails, no email domains, and no text the organizer wrote. The separate analytics reader allow-lists payload keys, so any new key has to be added there too.
• Docs: update docs/telemetry.md. Check whether the privacy page needs a line.
• The activity history view (No organizer-facing view of the activity log #243) will want to leave these events out, as it does page views.

Done when

• Each event is written by the action it describes, with a test.
• docs/telemetry.md lists the new events and their payloads.
• pnpm lint && pnpm typecheck && pnpm knip && pnpm test pass, and pnpm test:db passes for the activity writes.

Planned in https://claude.ai/code/session_01AkZbMUc4ej4ed7o5uh6nqr