feat(webhooks): enrich event.* payloads with customer contact + share_token (#341)

The event.published webhook reporter wired into n8n to send WhatsApp
gallery links was missing the data needed to actually message the
customer — only event_name + share_url were in the payload, no
customer_name / customer_email / customer_phone, and no bare share
token to construct alternate URLs.

Adds a single canonical event subject helper (webhookService.buildEventSubject)
so every event.* webhook returns the same shape:

  { id, slug, event_name, event_type, event_date,
    share_url, share_token,
    customer_name, customer_email, customer_phone }

Fields the caller does not have in scope come back as null — keys are
always present so receivers do not have to distinguish "field missing"
from "field null". Pure addition: existing receivers continue to work,
existing templates ${data.event.event_name} keep working, and new
templates can now reference ${data.event.customer_phone} etc.

Wired into all five firing sites:
- routes/events.js — public event create (created + published)
- routes/adminEvents.js — admin create + draft→publish
- routes/v1/events.js — public v1 API (created + published)
- services/expirationChecker.js — event.expired (extra: expires_at)
- services/archiveService.js — event.archived (extra: archive_path)

PII surface area widens (customer email/phone now flow to webhook
receivers), so:
- Settings → Webhooks UI gets an amber Callout above the create form
  warning admins to only point webhooks at receivers they trust.
- Docs page updated with the new payload sample, the always-present
  null contract, and a Callout warning.

Verified end-to-end against the local dev webhook receiver — delivered
payload contains all 10 fields. webhookDelivery integration suite
remains 8/8 green.

backend/src/services/archiveService.js

@@ -99,10 +99,26 @@ async function archiveEvent(event) {
     // Fire event.archived webhook (#327). Receivers infer per-photo loss
     // from this event — we deliberately do NOT fire photo.deleted for each
     // archived photo to avoid flooding subscribers on bulk archives.
+    // Canonical event subject (#341) so the shape matches event.created /
+    // event.published / event.expired; archive_path is an event.archived-
+    // specific extra.
     try {
       const webhookService = require('./webhookService');
       await webhookService.fire('event.archived', {
-        event: { id: event.id, slug: event.slug, event_name: event.event_name, archive_path: archiveRelKey },
+        event: {
+          ...webhookService.buildEventSubject({
+            id: event.id,
+            slug: event.slug,
+            event_name: event.event_name,
+            event_type: event.event_type,
+            event_date: event.event_date,
+            share_token: event.share_token,
+            customer_name: event.customer_name || event.host_name,
+            customer_email: event.customer_email || event.host_email,
+            customer_phone: event.customer_phone,
+          }),
+          archive_path: archiveRelKey,
+        },
       });
     } catch (e) { /* non-fatal */ }