mbproxy: add keepalive / connection monitoring

The DL205/DL260 ECOM emits no TCP keepalives, so an idle backend socket
can be silently dropped by a middlebox (switch, firewall, NAT) after
2-5 minutes. Enable OS SO_KEEPALIVE on backend and accepted upstream
sockets, and drive a periodic synthetic FC03 heartbeat on each idle
backend socket so a dead path is detected before a real client request
hits it. Controlled by Connection.Keepalive (ON by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Admin/StatusDto.cs

@@ -103,7 +103,13 @@ public sealed record PlcBackendStatus(
     long CacheMissCount,
     long CacheInvalidations,
     long CacheEntryCount,
-    long CacheBytes);
+    long CacheBytes,
+    /// <summary>Backend keepalive heartbeat probes issued on idle backend sockets.</summary>
+    long BackendHeartbeatsSent,
+    /// <summary>Keepalive heartbeat probes that timed out (backend not answering).</summary>
+    long BackendHeartbeatsFailed,
+    /// <summary>Backend teardowns triggered by a failed keepalive heartbeat.</summary>
+    long BackendIdleDisconnects);
 
 /// <summary>Modbus exception counts by code.</summary>
 public sealed record ExceptionCounts(