feat(api): add Grafana test endpoint for table format

Add `/api/v2/test/grafana-table` endpoint to validate Grafana
table format compatibility before implementing the full time
range API.

- Create server/grafana.go with table format structures
- Add structured logging and OpenTelemetry tracing
- Include realistic NTP Pool sample data with null handling
- Set proper CORS and cache headers for testing
- Update implementation plan with Phase 0 completion status

Ready for Grafana JSON API data source integration testing.

server/grafana.go

@@ -0,0 +1,100 @@
+package server
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/labstack/echo/v4"
+	"go.ntppool.org/common/logger"
+	"go.ntppool.org/common/tracing"
+)
+
+// ColumnDef represents a Grafana table column definition
+type ColumnDef struct {
+	Text string `json:"text"`
+	Type string `json:"type"`
+	Unit string `json:"unit,omitempty"`
+}
+
+// GrafanaTableSeries represents a single table series in Grafana format
+type GrafanaTableSeries struct {
+	Target  string            `json:"target"`
+	Tags    map[string]string `json:"tags"`
+	Columns []ColumnDef       `json:"columns"`
+	Values  [][]interface{}   `json:"values"`
+}
+
+// GrafanaTimeSeriesResponse represents the complete Grafana table response
+type GrafanaTimeSeriesResponse []GrafanaTableSeries
+
+// testGrafanaTable returns sample data in Grafana table format for validation
+func (srv *Server) testGrafanaTable(c echo.Context) error {
+	log := logger.Setup()
+	ctx, span := tracing.Tracer().Start(c.Request().Context(), "testGrafanaTable")
+	defer span.End()
+
+	log.InfoContext(ctx, "serving test Grafana table format", 
+		"remote_ip", c.RealIP(),
+		"user_agent", c.Request().UserAgent(),
+	)
+
+	// Generate sample data with realistic NTP Pool values
+	now := time.Now()
+	sampleData := GrafanaTimeSeriesResponse{
+		{
+			Target: "monitor{name=zakim1-yfhw4a}",
+			Tags: map[string]string{
+				"monitor_id":   "126",
+				"monitor_name": "zakim1-yfhw4a",
+				"type":         "monitor",
+				"status":       "active",
+			},
+			Columns: []ColumnDef{
+				{Text: "time", Type: "time"},
+				{Text: "score", Type: "number"},
+				{Text: "rtt", Type: "number", Unit: "ms"},
+				{Text: "offset", Type: "number", Unit: "s"},
+			},
+			Values: [][]interface{}{
+				{now.Add(-10*time.Minute).Unix() * 1000, 20.0, 18.865, -0.000267},
+				{now.Add(-20*time.Minute).Unix() * 1000, 20.0, 18.96, -0.000390},
+				{now.Add(-30*time.Minute).Unix() * 1000, 20.0, 18.073, -0.000768},
+				{now.Add(-40*time.Minute).Unix() * 1000, 20.0, 18.209, nil}, // null offset example
+			},
+		},
+		{
+			Target: "monitor{name=nj2-mon01}",
+			Tags: map[string]string{
+				"monitor_id":   "84",
+				"monitor_name": "nj2-mon01",
+				"type":         "monitor",
+				"status":       "active",
+			},
+			Columns: []ColumnDef{
+				{Text: "time", Type: "time"},
+				{Text: "score", Type: "number"},
+				{Text: "rtt", Type: "number", Unit: "ms"},
+				{Text: "offset", Type: "number", Unit: "s"},
+			},
+			Values: [][]interface{}{
+				{now.Add(-10*time.Minute).Unix() * 1000, 19.5, 22.145, 0.000123},
+				{now.Add(-20*time.Minute).Unix() * 1000, 19.8, 21.892, 0.000089},
+				{now.Add(-30*time.Minute).Unix() * 1000, 20.0, 22.034, 0.000156},
+			},
+		},
+	}
+
+	// Add CORS header for browser testing
+	c.Response().Header().Set("Access-Control-Allow-Origin", "*")
+	c.Response().Header().Set("Content-Type", "application/json")
+	
+	// Set cache control similar to other endpoints
+	c.Response().Header().Set("Cache-Control", "public,max-age=60")
+
+	log.InfoContext(ctx, "test Grafana table response sent", 
+		"series_count", len(sampleData),
+		"response_size_approx", "~1KB",
+	)
+
+	return c.JSON(http.StatusOK, sampleData)
+}