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>

internal/grpcserver/grpcserver_test.go

@@ -676,3 +676,62 @@ func TestGetStatusProxyProtocol(t *testing.T) {
 		t.Fatal("expected proxy_protocol = false")
 	}
 }
+
+func TestSetListenerMaxConnections(t *testing.T) {
+	env := setup(t)
+	ctx := context.Background()
+
+	// Set max connections.
+	_, err := env.client.SetListenerMaxConnections(ctx, &pb.SetListenerMaxConnectionsRequest{
+		ListenerAddr:   ":443",
+		MaxConnections: 5000,
+	})
+	if err != nil {
+		t.Fatalf("SetListenerMaxConnections: %v", err)
+	}
+
+	// Verify via GetStatus.
+	resp, err := env.client.GetStatus(ctx, &pb.GetStatusRequest{})
+	if err != nil {
+		t.Fatalf("GetStatus: %v", err)
+	}
+	if resp.Listeners[0].MaxConnections != 5000 {
+		t.Fatalf("max_connections = %d, want 5000", resp.Listeners[0].MaxConnections)
+	}
+
+	// Verify DB persistence.
+	l, _ := env.store.GetListenerByAddr(":443")
+	if l.MaxConnections != 5000 {
+		t.Fatalf("DB max_connections = %d, want 5000", l.MaxConnections)
+	}
+
+	// Set to 0 (unlimited).
+	_, err = env.client.SetListenerMaxConnections(ctx, &pb.SetListenerMaxConnectionsRequest{
+		ListenerAddr:   ":443",
+		MaxConnections: 0,
+	})
+	if err != nil {
+		t.Fatalf("SetListenerMaxConnections to 0: %v", err)
+	}
+
+	resp, _ = env.client.GetStatus(ctx, &pb.GetStatusRequest{})
+	if resp.Listeners[0].MaxConnections != 0 {
+		t.Fatalf("max_connections = %d, want 0", resp.Listeners[0].MaxConnections)
+	}
+}
+
+func TestSetListenerMaxConnectionsNotFound(t *testing.T) {
+	env := setup(t)
+	ctx := context.Background()
+
+	_, err := env.client.SetListenerMaxConnections(ctx, &pb.SetListenerMaxConnectionsRequest{
+		ListenerAddr:   ":9999",
+		MaxConnections: 100,
+	})
+	if err == nil {
+		t.Fatal("expected error for nonexistent listener")
+	}
+	if s, ok := status.FromError(err); !ok || s.Code() != codes.NotFound {
+		t.Fatalf("expected NotFound, got %v", err)
+	}
+}