Make AddRoute idempotent (upsert instead of reject duplicates)

AddRoute now updates an existing route if one already exists for the
same (listener, hostname) pair, instead of returning AlreadyExists.
This makes repeated deploys idempotent — the MCP agent can register
routes on every deploy without needing to remove them first.

- DB: INSERT ... ON CONFLICT DO UPDATE (SQLite upsert)
- In-memory: overwrite existing route unconditionally
- gRPC: error code changed from AlreadyExists to Internal (for real DB errors)

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

internal/grpcserver/grpcserver.go

@@ -144,10 +144,10 @@ func (a *AdminServer) AddRoute(_ context.Context, req *pb.AddRouteRequest) (*pb.
 		}
 	}
 
-	// Write-through: DB first, then memory.
+	// Write-through: DB first (upsert), then memory.
 	if _, err := a.store.CreateRoute(ls.ID, hostname, req.Route.Backend, mode,
 		req.Route.TlsCert, req.Route.TlsKey, req.Route.BackendTls, req.Route.SendProxyProtocol); err != nil {
-		return nil, status.Errorf(codes.AlreadyExists, "%v", err)
+		return nil, status.Errorf(codes.Internal, "%v", err)
 	}
 
 	info := server.RouteInfo{
@@ -158,10 +158,7 @@ func (a *AdminServer) AddRoute(_ context.Context, req *pb.AddRouteRequest) (*pb.
 		BackendTLS:        req.Route.BackendTls,
 		SendProxyProtocol: req.Route.SendProxyProtocol,
 	}
-	if err := ls.AddRoute(hostname, info); err != nil {
-		// DB succeeded but memory failed (should not happen since DB enforces uniqueness).
-		a.logger.Error("inconsistency: DB write succeeded but memory update failed", "error", err)
-	}
+	ls.AddRoute(hostname, info)
 
 	a.logger.Info("route added", "listener", ls.Addr, "hostname", hostname, "backend", req.Route.Backend, "mode", mode)
 	return &pb.AddRouteResponse{}, nil