feat(leads): implement Stripe payment component for exhibit licenses

Full implementation of ae_comp__exhibit_payment.svelte (was a 9-line stub).
Reads Stripe config from $ae_loc.site_cfg_json per-event. License tier
selector (1/3/6/10 users) uses {#key} remount pattern to work around
stripe-buy-button web component ignoring attribute changes after mount.
Three states: paid confirmation (priority=true), not-configured hint, payment
form. client_reference_id=exhibit_id ties payments to booth records.
TypeScript declaration for stripe-buy-button added to app.d.ts via
svelte/elements augmentation. exhibit_id prop wired in +page.svelte and
ae_tab__manage.svelte.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/MODULE__AE_Events_Exhibitor_Leads.md

@@ -260,3 +260,10 @@ Guard in [ae_tab__manage.svelte](src/routes/events/[event_id]/(leads)/leads/exhi
 - Export endpoint: `GET /v3/action/event_exhibit/{id}/tracking_export` — requires `leads_api_access`
 - Custom questions are stored per-exhibit in `leads_custom_questions_json` (not global)
 - The exhibitor landing page link format: `/events/[event_id]/leads/exhibit/[exhibit_exhibit_id]/`
+
+
+## Old Files for Reference
+
+@backups/legacy/events_leads_v2/exhibit/[slug]/+page.svelte
+@backups/legacy/events_leads_v2/exhibit/[slug]/leads_manage.svelte
+@backups/legacy/events_leads_v2/exhibit/[slug]/leads_payment.svelte