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/StatusSnapshotBuilder.cs

@@ -108,7 +108,10 @@ internal sealed class StatusSnapshotBuilder
                             CacheInvalidations:        0,
                             CacheEntryCount:           0,
                             CacheBytes:                0,
-                            ResponseDropForFullUpstream: 0);
+                            ResponseDropForFullUpstream: 0,
+                            BackendHeartbeatsSent:     0,
+                            BackendHeartbeatsFailed:   0,
+                            BackendIdleDisconnects:    0);
 
             long connectsSuccess = counters.ConnectsSuccess;
             long connectsFailed  = counters.ConnectsFailed;
@@ -152,7 +155,10 @@ internal sealed class StatusSnapshotBuilder
                     CacheMissCount:     counters.CacheMissCount,
                     CacheInvalidations: counters.CacheInvalidations,
                     CacheEntryCount:    counters.CacheEntryCount,
-                    CacheBytes:         counters.CacheBytes),
+                    CacheBytes:         counters.CacheBytes,
+                    BackendHeartbeatsSent:   counters.BackendHeartbeatsSent,
+                    BackendHeartbeatsFailed: counters.BackendHeartbeatsFailed,
+                    BackendIdleDisconnects:  counters.BackendIdleDisconnects),
                 Bytes: new PlcBytesStatus(
                     UpstreamIn:  counters.BytesUpstreamIn,
                     UpstreamOut: counters.BytesUpstreamOut)));