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/java/de/fete/domain/model/EventToken.java

@@ -0,0 +1,18 @@
+package de.fete.domain.model;
+
+import java.util.Objects;
+import java.util.UUID;
+
+/** Type-safe wrapper for the public event token. */
+public record EventToken(UUID value) {
+
+  /** Validates that the token value is not null. */
+  public EventToken {
+    Objects.requireNonNull(value, "eventToken must not be null");
+  }
+
+  /** Generates a new random event token. */
+  public static EventToken generate() {
+    return new EventToken(UUID.randomUUID());
+  }
+}