Growth tools: Webinars

Webinars are a sys-operated growth tool — operators run them on behalf of the platform, the public pages from F19.16 are how prospects discover them, and the data lives in the same database as the rest of the platform. The list/create/detail/registrations/interest views all sit under `sys/webinars` and require `sys_support` for mutations. Creating a webinar via `POST /sys/webinars` auto-provisions a Google Calendar event and a Meet URL by impersonating `host_email` through a Service Account with domain-wide delegation; on provisioning failure the webinar persists as `DRAFT` and the response includes a `meet_provision_error` field so the operator can fix credentials and retry via PATCH.

Patching the webinar issues a Calendar update with `sendUpdates="all"` whenever timing, host, title, or description changes, so attendees always see the canonical version; the database update is intentionally not rolled back on Calendar failure so the operator can manually retry without losing edits. Lifecycle actions enforce state transitions: `publish` (DRAFT → SCHEDULED) refuses without a `meet_url`; `cancel` emails attendees, deletes the Calendar event with `sendUpdates="all"`, and flips `promote_on_homepage=false`; `recording` (ENDED → PUBLISHED) attaches the recording URL and flips homepage promotion off; `delete` is allowed only from DRAFT. Attendee management surfaces the full attendee list, a UTF-8-BOM CSV export, an idempotent send-reminder endpoint with 24h and 1h horizons (tracked by `reminder_*_sent_at` so a re-send never duplicates), and a bulk `mark-attendance` toggle.

Interest-lead preview reads `MarketingWebinarInterest` rows whose topics overlap the webinar's tags and whose subscribers are not unsubscribed — the audience the webinar-post-event-dispatch fan-out will hit.