feat: add CalendarEventBuilder with shared event/transport logic

Services/CalendarEventBuilder.cs

@@ -0,0 +1,53 @@
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public record CalendarEventData(string Summary, string Description);
+
+public static class CalendarEventBuilder
+{
+    public static CalendarEventData BuildEventContent(Shift shift, TransportConnection? transport)
+    {
+        var description = transport != null
+            ? $"Transport: {FormatTransportInfo(transport)}"
+            : string.Empty;
+        return new CalendarEventData(shift.Name, description);
+    }
+
+    public static string FormatTransportInfo(TransportConnection transport)
+    {
+        var departure = transport.DepartureTime.ToString("HH:mm");
+        var arrival = transport.ArrivalTime.ToString("HH:mm");
+        return $"{transport.Platform}: {departure} → {arrival} ({transport.Duration.TotalMinutes} Minutes)";
+    }
+
+    public static async Task<T> ExecuteWithRetryAsync<T>(Func<Task<T>> operation, int maxRetries = 3)
+    {
+        var delay = TimeSpan.FromSeconds(1);
+
+        for (var attempt = 0; attempt <= maxRetries; attempt++)
+        {
+            try
+            {
+                return await operation();
+            }
+            catch (HttpRequestException ex) when (ex.Message.Contains("429") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Rate Limit") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Forbidden") && ex.Message.Contains("quota") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+        }
+
+        return await operation();
+    }
+}

ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs

@@ -0,0 +1,69 @@
+using ShiftScheduler.Services;
+using ShiftScheduler.Shared;
+using Shouldly;
+
+namespace ShiftScheduler.Services.Tests;
+
+public class CalendarEventBuilderTests
+{
+    [Fact]
+    public void BuildEventContent_WithTransport_IncludesTransportInDescription()
+    {
+        var shift = new Shift { Name = "Früh" };
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, transport);
+
+        result.Summary.ShouldBe("Früh");
+        result.Description.ShouldBe("Transport: Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void BuildEventContent_WithoutTransport_EmptyDescription()
+    {
+        var shift = new Shift { Name = "Spät" };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, null);
+
+        result.Summary.ShouldBe("Spät");
+        result.Description.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void FormatTransportInfo_FormatsCorrectly()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe("Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void FormatTransportInfo_WithNullPlatform_UsesEmptyString()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = null,
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe(": 05:30 → 05:55 (25 Minutes)");
+    }
+}