Add RSVP creation endpoint with typed tokens and attendee count

Introduce typed token value objects (EventToken, OrganizerToken,
RsvpToken) and refactor all existing Event code to use them.

Add POST /events/{token}/rsvps endpoint that persists an RSVP and
returns an rsvpToken. Populate attendeeCount in GET /events/{token}
from a real count query instead of hardcoded 0.

Includes: OpenAPI spec, Liquibase migration (rsvps table with
ON DELETE CASCADE), domain model, hexagonal ports/adapters,
service layer, and full test coverage (unit + integration).

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

backend/src/main/resources/db/changelog/003-create-rsvps-table.xml

@@ -0,0 +1,36 @@
+<?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="003-create-rsvps-table" author="fete">
+        <createTable tableName="rsvps">
+            <column name="id" type="bigserial" autoIncrement="true">
+                <constraints primaryKey="true" nullable="false"/>
+            </column>
+            <column name="rsvp_token" type="uuid">
+                <constraints nullable="false" unique="true"/>
+            </column>
+            <column name="event_id" type="bigint">
+                <constraints nullable="false"
+                             foreignKeyName="fk_rsvps_event_id"
+                             references="events(id)"
+                             deleteCascade="true"/>
+            </column>
+            <column name="name" type="varchar(100)">
+                <constraints nullable="false"/>
+            </column>
+        </createTable>
+
+        <createIndex tableName="rsvps" indexName="idx_rsvps_event_id">
+            <column name="event_id"/>
+        </createIndex>
+
+        <createIndex tableName="rsvps" indexName="idx_rsvps_rsvp_token">
+            <column name="rsvp_token"/>
+        </createIndex>
+    </changeSet>
+
+</databaseChangeLog>