plan/10: Add /api/stats endpoint and dashboard stats badge (#2)

Adds GET /api/stats endpoint with agent/plan/task/queue counts, stats widget in dashboard header, and Go test coverage.

api/handlers/stats_test.go

@@ -0,0 +1,39 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestGetStats(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("GET", "/api/stats", nil)
+
+	// Create a mock handler with nil DB and PubSub (since we're not testing the actual DB queries)
+	h := &Handler{db: nil, ps: nil}
+
+	h.GetStats(c)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d", w.Code)
+	}
+
+	var body map[string]any
+	if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+		t.Fatalf("invalid json: %v", err)
+	}
+
+	// Check that all expected top-level fields are present
+	expectedFields := []string{"agent_count", "plan_counts", "task_counts", "queue_counts"}
+	for _, field := range expectedFields {
+		if _, ok := body[field]; !ok {
+			t.Errorf("missing expected field: %s", field)
+		}
+	}
+}