Add POST /api/admin/reset-latency endpoint

Zeroes block-update latency counters (count, sum, last, wasted work)
in both memory and the kv store. Used by the StartOS reset-latency
action to let operators start fresh after tuning.

api/internal/httpapi/server.go

@@ -38,6 +38,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /api/snapshot", s.snapshot)
 	mux.HandleFunc("GET /api/ws", s.handleWS)
 	mux.HandleFunc("GET /api/admin/debug-blocks", s.debugBlocks)
+	mux.HandleFunc("POST /api/admin/reset-latency", s.resetLatency)
 	mux.Handle("/", spaHandler(webui.FS()))
 	return mux
 }
@@ -208,3 +209,8 @@ func (s *Server) debugBlocks(w http.ResponseWriter, r *http.Request) {
 		"db":     db,
 	})
 }
+
+func (s *Server) resetLatency(w http.ResponseWriter, _ *http.Request) {
+	s.Agg.ResetLatency()
+	writeJSON(w, http.StatusOK, map[string]any{"ok": true})
+}