fix: respect optional email settings in event creation (#217)

When admin/customer emails were configured as optional in Settings >
Event Creation, the backend still rejected empty values because:

1. express-validator .optional() only skips undefined, not empty strings
   — changed to .optional({ values: 'falsy' }) so "" is treated as
   absent
2. DB columns host_email and admin_email had NOT NULL constraints
   — added migration to make them nullable
3. Email queue insert crashed on null recipient_email
   — skip queuing when no customer email is provided

backend/migrations/core/073_make_event_emails_nullable.js

@@ -0,0 +1,13 @@
+exports.up = async function(knex) {
+  await knex.schema.alterTable('events', (table) => {
+    table.string('host_email', 255).nullable().alter();
+    table.string('admin_email', 255).nullable().alter();
+  });
+};
+
+exports.down = async function(knex) {
+  await knex.schema.alterTable('events', (table) => {
+    table.string('host_email', 255).notNullable().defaultTo('').alter();
+    table.string('admin_email', 255).notNullable().defaultTo('').alter();
+  });
+};