Fix: Outbox relay has no dead-letter or max retry limit + DataLake writer partial commit inconsistency + ThresholdCacheLoader crashes startup on Redis failure

VigilCareClinicalAPI/BackgroundServices/ThresholdCacheLoader.cs

@@ -22,26 +22,51 @@ public class ThresholdCacheLoader : IHostedService
     {
         using var scope = _services.CreateScope();
         var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
-        var cache = _redis.GetDatabase();
 
         var thresholds = await db.AlertThresholds.ToListAsync(cancellationToken);
-        var batch = cache.CreateBatch();
 
-        foreach (var t in thresholds)
+        const int maxAttempts = 3;
+        int[] backoffMs = [2000, 4000, 8000];
+
+        for (var attempt = 0; attempt < maxAttempts; attempt++)
         {
-            var json = JsonSerializer.Serialize(new
+            try
             {
-                t.ObservationCode,
-                t.CriticalLow,
-                t.WarningLow,
-                t.WarningHigh,
-                t.CriticalHigh
-            });
-            _ = batch.StringSetAsync($"threshold:{t.ObservationCode}", json);
+                var cache = _redis.GetDatabase();
+                var batch = cache.CreateBatch();
+
+                foreach (var t in thresholds)
+                {
+                    var json = JsonSerializer.Serialize(new
+                    {
+                        t.ObservationCode,
+                        t.CriticalLow,
+                        t.WarningLow,
+                        t.WarningHigh,
+                        t.CriticalHigh
+                    });
+                    _ = batch.StringSetAsync($"threshold:{t.ObservationCode}", json);
+                }
+
+                batch.Execute();
+                _logger.LogInformation("Loaded {Count} alert thresholds into Redis cache", thresholds.Count);
+                return;
+            }
+            catch (RedisException ex)
+            {
+                _logger.LogWarning(ex,
+                    "Redis unavailable during threshold cache load — attempt {Attempt}/{Max}",
+                    attempt + 1, maxAttempts);
+
+                if (attempt < maxAttempts - 1)
+                    await Task.Delay(backoffMs[attempt], cancellationToken);
+            }
         }
 
-        batch.Execute();
-        _logger.LogInformation("Loaded {Count} alert thresholds into Redis cache", thresholds.Count);
+        _logger.LogError(
+            "Failed to load thresholds into Redis after {Max} attempts — " +
+            "application will start without cache; observation ingest falls back to PostgreSQL",
+            maxAttempts);
     }
 
     public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;