Add GET /events/{token} endpoint and timezone field to OpenAPI spec

OpenAPI: new GetEventResponse schema, timezone on Create request/response.
Liquibase: add timezone VARCHAR(64) NOT NULL DEFAULT 'UTC' column.

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

backend/src/main/resources/db/changelog/002-add-timezone-column.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<databaseChangeLog
+    xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
+        http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-latest.xsd">
+
+    <changeSet id="002-add-timezone-column" author="fete">
+        <addColumn tableName="events">
+            <column name="timezone" type="varchar(64)" defaultValue="UTC">
+                <constraints nullable="false"/>
+            </column>
+        </addColumn>
+    </changeSet>
+
+</databaseChangeLog>