fix: handle Nextcloud CalDAV 429 rate limiting with Polly retry

Replaces hand-rolled retry logic with Microsoft.Extensions.Http.Resilience
(Polly v8). The named 'nextcloud' HttpClient retries up to 3 times on 429
with exponential backoff (2s, 4s, 8s). Also adds a 300ms delay after every
individual PUT/DELETE request to reduce burst rate.

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

Server/Program.cs

@@ -1,6 +1,9 @@
 using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.AspNetCore.Authentication.Google;
 using Microsoft.AspNetCore.Authorization;
+using Microsoft.Extensions.Http.Resilience;
+using Polly;
+using System.Net;
 using System.Security.Claims;
 using ShiftScheduler.Services;
 using ShiftScheduler.Shared;
@@ -24,6 +27,18 @@ builder.Services.AddSingleton(authorizedEmails);
 builder.Services.AddSingleton<IConfigurationService>(new ConfigurationService(appConfiguration));
 builder.Services.AddMemoryCache();
 builder.Services.AddHttpClient<TransportApiService>();
+builder.Services.AddHttpClient("nextcloud")
+    .AddResilienceHandler("nextcloud-retry", pipeline =>
+    {
+        pipeline.AddRetry(new HttpRetryStrategyOptions
+        {
+            MaxRetryAttempts = 3,
+            Delay = TimeSpan.FromSeconds(2),
+            BackoffType = DelayBackoffType.Exponential,
+            ShouldHandle = args => ValueTask.FromResult(
+                args.Outcome.Result?.StatusCode == HttpStatusCode.TooManyRequests)
+        });
+    });
 builder.Services.AddSingleton<IcsExportService>();
 builder.Services.AddSingleton<PdfExportService>();
 builder.Services.AddSingleton<ITransportApiService, TransportApiService>();

Server/ShiftScheduler.Server.csproj

@@ -3,6 +3,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.Google" Version="9.0.8" />
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Server" Version="9.0.8" />
+    <PackageReference Include="Microsoft.Extensions.Http.Resilience" Version="9.0.0" />
   </ItemGroup>
 
   <ItemGroup>

Services/NextcloudCalendarService.cs

@@ -110,7 +110,7 @@ public class NextcloudCalendarService(
         reportRequest.Headers.Add("Depth", "1");
         reportRequest.Content = new StringContent(body, Encoding.UTF8, "application/xml");
 
-        var client = httpClientFactory.CreateClient();
+        var client = httpClientFactory.CreateClient("nextcloud");
         var reportResponse = await client.SendAsync(reportRequest);
         reportResponse.EnsureSuccessStatusCode();
 
@@ -126,8 +126,7 @@ public class NextcloudCalendarService(
                 deleteResponse.StatusCode != System.Net.HttpStatusCode.NotFound)
                 deleteResponse.EnsureSuccessStatusCode();
 
-            if (eventUrls.Count > 1)
+            await Task.Delay(300);
-                await Task.Delay(200);
         }
     }
 
@@ -155,7 +154,7 @@ public class NextcloudCalendarService(
 
     private async Task CreateShiftEventsAsync(string calendarId, List<ShiftWithTransport> shifts)
     {
-        var client = httpClientFactory.CreateClient();
+        var client = httpClientFactory.CreateClient("nextcloud");
 
         foreach (var shiftWithTransport in shifts)
         {
@@ -176,6 +175,7 @@ public class NextcloudCalendarService(
                 var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
                 putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
                 (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
+                await Task.Delay(300);
             }
 
             if (!string.IsNullOrEmpty(shift.AfternoonTime))
@@ -189,10 +189,8 @@ public class NextcloudCalendarService(
                 var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
                 putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
                 (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
-            }
-
-            if (shifts.Count > 1)
                 await Task.Delay(300);
+            }
         }
     }