Add per-listener connection limits

Configurable maximum concurrent connections per listener. When the
limit is reached, new connections are closed immediately after accept.
0 means unlimited (default, preserving existing behavior).

Config: Listener gains max_connections field, validated non-negative.

DB: Migration 3 adds listeners.max_connections column.
UpdateListenerMaxConns method for runtime changes via gRPC.
CreateListener updated to persist max_connections on seed.

Server: ListenerState/ListenerData gain MaxConnections. Limit checked
in serve() after Accept but before handleConn — if ActiveConnections
>= MaxConnections, connection is closed and the accept loop continues.
SetMaxConnections method for runtime updates.

Proto: SetListenerMaxConnections RPC added. ListenerStatus gains
max_connections field. Generated code regenerated.

gRPC server: SetListenerMaxConnections implements write-through
(DB first, then in-memory update). GetStatus includes max_connections.

Client: SetListenerMaxConnections method, MaxConnections in
ListenerStatus.

Tests: DB CRUD and UpdateListenerMaxConns, server connection limit
enforcement (accept 2, reject 3rd, close one, accept again), gRPC
SetListenerMaxConnections round-trip with DB persistence, not-found
error handling.

Also updates PROJECT_PLAN.md with phases 6-8 and PROGRESS.md with
tracking for the new features.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/mcproxy/client.go

@@ -161,6 +161,7 @@ type ListenerStatus struct {
 	RouteCount        int
 	ActiveConnections int64
 	ProxyProtocol     bool
+	MaxConnections    int64
 }
 
 // Status contains the server's current status.
@@ -193,12 +194,23 @@ func (c *Client) GetStatus(ctx context.Context) (*Status, error) {
 			RouteCount:        int(ls.RouteCount),
 			ActiveConnections: ls.ActiveConnections,
 			ProxyProtocol:     ls.ProxyProtocol,
+			MaxConnections:    ls.MaxConnections,
 		}
 	}
 
 	return status, nil
 }
 
+// SetListenerMaxConnections updates the per-listener connection limit.
+// 0 means unlimited.
+func (c *Client) SetListenerMaxConnections(ctx context.Context, listenerAddr string, maxConns int64) error {
+	_, err := c.admin.SetListenerMaxConnections(ctx, &pb.SetListenerMaxConnectionsRequest{
+		ListenerAddr:   listenerAddr,
+		MaxConnections: maxConns,
+	})
+	return err
+}
+
 // HealthStatus represents the health of the server.
 type HealthStatus int