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/webhookService.js

@@ -210,6 +210,29 @@ function parseJsonField(value, fallback) {
   try { return JSON.parse(value) ?? fallback; } catch { return fallback; }
 }
 
+/**
+ * Canonical event sub-object for outbound webhooks (#341). Always returns
+ * the full key set so receivers don't have to handle "field missing vs
+ * field null" — pass whatever the caller has in scope, missing fields
+ * become null. Adding a new field here propagates to every event.* type
+ * payload at once.
+ */
+function buildEventSubject(input = {}) {
+  const e = input || {};
+  return {
+    id: e.id ?? null,
+    slug: e.slug ?? null,
+    event_name: e.event_name ?? null,
+    event_type: e.event_type ?? null,
+    event_date: e.event_date ?? null,
+    share_url: e.share_url ?? null,
+    share_token: e.share_token ?? null,
+    customer_name: e.customer_name ?? null,
+    customer_email: e.customer_email ?? null,
+    customer_phone: e.customer_phone ?? null,
+  };
+}
+
 module.exports = {
   fire,
   generateSecret,
@@ -219,6 +242,7 @@ module.exports = {
   renderTemplate,
   validateTemplate,
   getByPath,
+  buildEventSubject,
   EVENT_TYPES,
   SECRET_PREFIX,
 };