Make expiryDate an internal concern, auto-set to event date + 7 days

The expiry date is no longer user-facing: it is removed from the API
(request and response) and the frontend. The backend now automatically
calculates it as the event date plus 7 days. The expired banner and
RSVP-bar filtering by expired status are also removed from the UI,
since expiry is purely an internal data-retention mechanism.

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

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

@@ -160,7 +160,6 @@ components:
         - title
         - dateTime
         - timezone
-        - expiryDate
       properties:
         title:
           type: string
@@ -181,11 +180,6 @@ components:
         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
@@ -195,7 +189,6 @@ components:
         - title
         - dateTime
         - timezone
-        - expiryDate
       properties:
         eventToken:
           type: string
@@ -218,10 +211,6 @@ components:
           type: string
           description: IANA timezone of the organizer
           example: "Europe/Berlin"
-        expiryDate:
-          type: string
-          format: date
-          example: "2026-06-15"
 
     GetEventResponse:
       type: object
@@ -231,7 +220,6 @@ components:
         - dateTime
         - timezone
         - attendeeCount
-        - expired
       properties:
         eventToken:
           type: string
@@ -264,10 +252,6 @@ components:
           minimum: 0
           description: Number of confirmed attendees (attending=true)
           example: 12
-        expired:
-          type: boolean
-          description: Whether the event's expiry date has passed
-          example: false
 
     CreateRsvpRequest:
       type: object