implemented ical exports

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

doc/openapi3_0.yaml

@@ -119,6 +119,42 @@ paths:
           description: server fault
       summary:
         store a new event in the database
+  /api/event/ical: 
+    get:
+      parameters:
+        - description: the appointment id
+          in: query
+          name: id
+          required: true
+          schema:
+            example: 42
+            format: int64
+            type: number
+      responses:
+        '200':
+          description: successful operation
+          content:
+            text/calendar:
+              schema:
+                description: event in ical format
+                example: |-
+                  BEGIN:VCALENDAR
+                  VERSION:2.0
+                  PRODID:OpenCloudCal
+                  BEGIN:VEVENT
+                  UID:42@cal.srsoftware.de
+                  DTSTART:20241230T100757Z
+                  DTEND:20241231T100757Z
+                  SUMMARY:Demolition of the earth
+                  END:VEVENT
+                  END:VCALENDAR
+                type: string
+        '400':
+          description: invalid input
+        '500':
+          description: server fault
+      summary: get an event as ical file
+ 
   /api/events/ical:
     get:
       description: |-