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/db.changelog-master.xml

@@ -8,5 +8,6 @@
     <include file="db/changelog/000-baseline.xml"/>
     <include file="db/changelog/001-create-events-table.xml"/>
     <include file="db/changelog/002-add-timezone-column.xml"/>
+    <include file="db/changelog/003-create-rsvps-table.xml"/>
 
 </databaseChangeLog>