Add 008-rsvp feature spec and design artifacts

Spec, research decisions, implementation plan, data model,
API contract, and task breakdown for the RSVP feature.

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

specs/008-rsvp/contracts/create-rsvp.yaml

@@ -0,0 +1,79 @@
+# OpenAPI contract addition for POST /events/{eventToken}/rsvps
+# To be merged into backend/src/main/resources/openapi/api.yaml
+
+paths:
+  /events/{eventToken}/rsvps:
+    post:
+      operationId: createRsvp
+      summary: Submit an RSVP for an event
+      tags:
+        - events
+      parameters:
+        - name: eventToken
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: Public event token
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/CreateRsvpRequest"
+      responses:
+        "201":
+          description: RSVP created successfully
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CreateRsvpResponse"
+        "400":
+          description: Validation failed (e.g. blank name)
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ValidationProblemDetail"
+        "404":
+          description: Event not found
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemDetail"
+        "409":
+          description: Event has expired — RSVPs no longer accepted
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemDetail"
+
+components:
+  schemas:
+    CreateRsvpRequest:
+      type: object
+      required:
+        - name
+      properties:
+        name:
+          type: string
+          minLength: 1
+          maxLength: 100
+          description: Guest's display name
+          example: "Max Mustermann"
+
+    CreateRsvpResponse:
+      type: object
+      required:
+        - rsvpToken
+        - name
+      properties:
+        rsvpToken:
+          type: string
+          format: uuid
+          description: Token identifying this RSVP (store client-side for future updates)
+          example: "d4e5f6a7-b8c9-0123-4567-890abcdef012"
+        name:
+          type: string
+          description: Guest's display name as stored
+          example: "Max Mustermann"