perf(store-and-forward): bound retry sweep with SweepBatchLimit (default 500) on the due-rows query

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardOptions.cs

@@ -22,4 +22,11 @@ public class StoreAndForwardOptions
 
     /// <summary>Interval for the background retry timer sweep.</summary>
     public TimeSpan RetryTimerInterval { get; set; } = TimeSpan.FromSeconds(10);
+
+    /// <summary>
+    /// Maximum due rows loaded per retry sweep. Bounds sweep memory and duration
+    /// under backlog; the remainder is picked up on the next tick (10 s).
+    /// 0 = unlimited (legacy).
+    /// </summary>
+    public int SweepBatchLimit { get; set; } = 500;
 }

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs

@@ -601,7 +601,7 @@ public class StoreAndForwardService
                 }
             }
 
-            var messages = await _storage.GetMessagesForRetryAsync();
+            var messages = await _storage.GetMessagesForRetryAsync(_options.SweepBatchLimit);
             if (messages.Count == 0) return;
 
             _logger.LogDebug("Retry sweep: {Count} messages due for retry", messages.Count);

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardStorage.cs

@@ -180,7 +180,7 @@ public class StoreAndForwardStorage
     /// Gets all messages that are due for retry (Pending status, last attempt older than retry interval).
     /// </summary>
     /// <returns>A task that resolves to the list of messages due for retry, ordered by creation time ascending.</returns>
-    public async Task<List<StoreAndForwardMessage>> GetMessagesForRetryAsync()
+    public async Task<List<StoreAndForwardMessage>> GetMessagesForRetryAsync(int limit = 0)
     {
         await using var connection = new SqliteConnection(_connectionString);
         await connection.OpenAsync();
@@ -197,6 +197,13 @@ public class StoreAndForwardStorage
                    OR (julianday('now') - julianday(last_attempt_at)) * 86400000 >= retry_interval_ms)
             ORDER BY created_at ASC";
 
+        // Bound the sweep batch (oldest-first). 0 = unlimited (legacy).
+        if (limit > 0)
+        {
+            cmd.CommandText += "\n            LIMIT @limit";
+            cmd.Parameters.AddWithValue("@limit", limit);
+        }
+
         cmd.Parameters.AddWithValue("@pending", (int)StoreAndForwardMessageStatus.Pending);
 
         return await ReadMessagesAsync(cmd);