Add post and event cards, shared calendar sheet and per-event ICS; rework past events and next-event card

Elternbeirat.Web/Program.cs

@@ -69,6 +69,28 @@ app.MapGet("/events.ics", async (PocketBaseClient pocketBase, CancellationToken
     return Results.Text(IcsCalendar.Build(events), "text/calendar; charset=utf-8");
 });
 
+// One event as a calendar file ("In Kalender übernehmen" on the events page): the
+// same calendar as the feed, holding just this event. Unlike the feed it does not
+// fall back to an empty calendar when PocketBase is down -- a file without the event
+// the visitor clicked would be a silent failure, so it answers 503. An unknown or
+// non-public id is a 404.
+app.MapGet("/events/{id}.ics", async (string id, PocketBaseClient pocketBase, CancellationToken token) =>
+{
+    IReadOnlyList<Event> events;
+    try
+    {
+        events = await pocketBase.GetEventsAsync(token);
+    }
+    catch (PocketBaseUnavailableException)
+    {
+        return Results.StatusCode(StatusCodes.Status503ServiceUnavailable);
+    }
+
+    return events.FirstOrDefault(@event => @event.Id == id) is { } match
+        ? Results.Text(IcsCalendar.Build([match]), "text/calendar; charset=utf-8")
+        : Results.NotFound();
+});
+
 // Status page for an outside monitor (Uptime Kuma) to check. One check tells all
 // three states apart: no reply = the app is down, 503 = the app runs but PocketBase
 // is down, 200 = both are fine. It talks to PocketBase directly, not through the