Add OpenAPI spec for POST /events endpoint

Define CreateEventRequest, CreateEventResponse, ProblemDetail, and
ValidationProblemDetail schemas. RFC 9457 problem details for errors.

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

backend/src/main/resources/openapi/api.yaml

@@ -11,27 +11,120 @@ servers:
   - url: /api
 
 paths:
-  /health:
+  /events:
-    get:
+    post:
-      operationId: getHealth
+      operationId: createEvent
-      summary: Health check
+      summary: Create a new event
       tags:
-        - health
+        - events
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/CreateEventRequest"
       responses:
-        "200":
+        "201":
-          description: Service is healthy
+          description: Event created successfully
           content:
             application/json:
               schema:
-                $ref: "#/components/schemas/HealthResponse"
+                $ref: "#/components/schemas/CreateEventResponse"
+        "400":
+          description: Validation failed
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ValidationProblemDetail"
 
 components:
   schemas:
-    HealthResponse:
+    CreateEventRequest:
       type: object
       required:
-        - status
+        - title
+        - dateTime
+        - expiryDate
       properties:
-        status:
+        title:
           type: string
-          example: UP
+          minLength: 1
+          maxLength: 200
+        description:
+          type: string
+          maxLength: 2000
+        dateTime:
+          type: string
+          format: date-time
+          description: Event date and time with UTC offset (ISO 8601)
+          example: "2026-03-15T20:00:00+01:00"
+        location:
+          type: string
+          maxLength: 500
+        expiryDate:
+          type: string
+          format: date
+          description: Date after which event data is deleted. Must be in the future.
+          example: "2026-06-15"
+
+    CreateEventResponse:
+      type: object
+      required:
+        - eventToken
+        - organizerToken
+        - title
+        - dateTime
+        - expiryDate
+      properties:
+        eventToken:
+          type: string
+          format: uuid
+          description: Public token for the event URL
+        organizerToken:
+          type: string
+          format: uuid
+          description: Secret token for organizer access
+        title:
+          type: string
+        dateTime:
+          type: string
+          format: date-time
+        expiryDate:
+          type: string
+          format: date
+
+    ProblemDetail:
+      type: object
+      properties:
+        type:
+          type: string
+          format: uri
+          default: "about:blank"
+        title:
+          type: string
+        status:
+          type: integer
+        detail:
+          type: string
+        instance:
+          type: string
+          format: uri
+      additionalProperties: true
+
+    ValidationProblemDetail:
+      allOf:
+        - $ref: "#/components/schemas/ProblemDetail"
+        - type: object
+          properties:
+            fieldErrors:
+              type: array
+              items:
+                type: object
+                required:
+                  - field
+                  - message
+                properties:
+                  field:
+                    type: string
+                  message:
+                    type: string