fix(discord): add backoff to scheduler to prevent 403 spam

- SessionSchedulerService now backs off for 15 minutes after any
  handler failure (confirmation, one-hour reminder, join link),
  preventing infinite retry loops on Discord 403 Missing Access.
- Added per-session ConcurrentDictionary backoff tracking with
  automatic cleanup on success.
- Enhanced DiscordPlatformMessenger logging for SendConfirmation
  and SendJoinLink to aid permission diagnostics.
- Added 3 regression tests for backoff behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Shared/Infrastructure/Scheduling/SessionSchedulerService.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using GmRelay.Shared.Features.Confirmation.SendConfirmation;
 using GmRelay.Shared.Features.Reminders.SendJoinLink;
 using GmRelay.Shared.Features.Reminders.SendOneHourReminder;
@@ -20,6 +21,11 @@ public sealed class SessionSchedulerService(
     ILogger<SessionSchedulerService> logger) : BackgroundService
 {
     private static readonly TimeSpan TickInterval = TimeSpan.FromMinutes(1);
+    private static readonly TimeSpan BackoffDuration = TimeSpan.FromMinutes(15);
+
+    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _confirmationBackoff = new();
+    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _oneHourBackoff = new();
+    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _joinLinkBackoff = new();
 
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
@@ -71,14 +77,30 @@ public sealed class SessionSchedulerService(
 
         foreach (var sessionId in sessionIds)
         {
+            if (_confirmationBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
+            {
+                logger.LogDebug(
+                    "Skipping confirmation for session {SessionId} until {Backoff}",
+                    sessionId,
+                    backoffUntil);
+                continue;
+            }
+
             try
             {
                 await confirmationHandler.HandleAsync(sessionId, ct);
+                _confirmationBackoff.TryRemove(sessionId, out _);
                 logger.LogInformation("Confirmation sent for session {SessionId}", sessionId);
             }
             catch (Exception ex)
             {
-                logger.LogError(ex, "Failed to send confirmation for session {SessionId}", sessionId);
+                var nextAttempt = now.Add(BackoffDuration);
+                _confirmationBackoff[sessionId] = nextAttempt;
+                logger.LogError(
+                    ex,
+                    "Failed to send confirmation for session {SessionId}, backing off until {Backoff}",
+                    sessionId,
+                    nextAttempt);
             }
         }
     }
@@ -98,14 +120,30 @@ public sealed class SessionSchedulerService(
 
         foreach (var sessionId in sessionIds)
         {
+            if (_oneHourBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
+            {
+                logger.LogDebug(
+                    "Skipping one-hour reminder for session {SessionId} until {Backoff}",
+                    sessionId,
+                    backoffUntil);
+                continue;
+            }
+
             try
             {
                 await oneHourReminderHandler.HandleAsync(sessionId, ct);
+                _oneHourBackoff.TryRemove(sessionId, out _);
                 logger.LogInformation("One-hour reminder processed for session {SessionId}", sessionId);
             }
             catch (Exception ex)
             {
-                logger.LogError(ex, "Failed to process one-hour reminder for session {SessionId}", sessionId);
+                var nextAttempt = now.Add(BackoffDuration);
+                _oneHourBackoff[sessionId] = nextAttempt;
+                logger.LogError(
+                    ex,
+                    "Failed to process one-hour reminder for session {SessionId}, backing off until {Backoff}",
+                    sessionId,
+                    nextAttempt);
             }
         }
     }
@@ -125,14 +163,30 @@ public sealed class SessionSchedulerService(
 
         foreach (var sessionId in sessionIds)
         {
+            if (_joinLinkBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
+            {
+                logger.LogDebug(
+                    "Skipping join link for session {SessionId} until {Backoff}",
+                    sessionId,
+                    backoffUntil);
+                continue;
+            }
+
             try
             {
                 await joinLinkHandler.HandleAsync(sessionId, ct);
+                _joinLinkBackoff.TryRemove(sessionId, out _);
                 logger.LogInformation("Join link sent for session {SessionId}", sessionId);
             }
             catch (Exception ex)
             {
-                logger.LogError(ex, "Failed to send join link for session {SessionId}", sessionId);
+                var nextAttempt = now.Add(BackoffDuration);
+                _joinLinkBackoff[sessionId] = nextAttempt;
+                logger.LogError(
+                    ex,
+                    "Failed to send join link for session {SessionId}, backing off until {Backoff}",
+                    sessionId,
+                    nextAttempt);
             }
         }
     }