feat(PROJ-40,PROJ-41): Prometheus Metriken + Dashboard Zeitreihe

- PROJ-40: /api/health mit Version+Uptime, /metrics Prometheus-Format
  (mails_last_60min/24h/7d/30d, mails_total, storage_bytes, tenants_total,
   users_total, uptime_seconds) — Token-Schutz optional konfigurierbar
- PROJ-41: GET /api/admin/stats/timeseries (30-Tage tagesgenau, Tenant-scoped)
  + SVG-Balkendiagramm im Dashboard (Mail-Eingang letzte 30 Tage)
- storage.DBQueryRow() Helper für Metrics-Queries ohne Pool-Exposition
- config.MetricsConfig (enabled, token) in config.go

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/storage/storage_stats.go

@@ -5,6 +5,73 @@ import (
 	"time"
 )
 
+// TimeseriesPoint holds mail count for a single day.
+type TimeseriesPoint struct {
+	Day   string `json:"day"`   // "2026-04-05"
+	Count int64  `json:"count"`
+}
+
+// MailTimeseries returns daily mail counts for the last `days` days.
+// Tenant-scoped when tenantID is non-nil.
+func (s *Store) MailTimeseries(ctx context.Context, days int, tenantID *int64) ([]TimeseriesPoint, error) {
+	if s.db == nil {
+		return nil, nil
+	}
+	if days <= 0 {
+		days = 30
+	}
+
+	var rows interface {
+		Next() bool
+		Scan(dest ...interface{}) error
+		Close()
+	}
+	var err error
+
+	if tenantID != nil {
+		rows, err = s.db.Query(ctx, `
+			SELECT date_trunc('day', e.received_at AT TIME ZONE 'UTC')::date AS day,
+			       COUNT(*) AS cnt
+			FROM emails e
+			JOIN email_refs r ON r.email_id = e.id AND r.tenant_id = $2
+			WHERE e.received_at >= NOW() - ($1 || ' days')::interval
+			GROUP BY day
+			ORDER BY day ASC
+		`, days, *tenantID)
+	} else {
+		rows, err = s.db.Query(ctx, `
+			SELECT date_trunc('day', received_at AT TIME ZONE 'UTC')::date AS day,
+			       COUNT(*) AS cnt
+			FROM emails
+			WHERE received_at >= NOW() - ($1 || ' days')::interval
+			GROUP BY day
+			ORDER BY day ASC
+		`, days)
+	}
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	// Build a map day→count then fill in zeros for missing days
+	counts := map[string]int64{}
+	for rows.Next() {
+		var day time.Time
+		var cnt int64
+		if err := rows.Scan(&day, &cnt); err == nil {
+			counts[day.Format("2006-01-02")] = cnt
+		}
+	}
+
+	result := make([]TimeseriesPoint, days)
+	for i := range result {
+		d := time.Now().UTC().AddDate(0, 0, -(days-1-i))
+		key := d.Format("2006-01-02")
+		result[i] = TimeseriesPoint{Day: key, Count: counts[key]}
+	}
+	return result, nil
+}
+
 // MailActivity holds mail counts over different time windows.
 type MailActivity struct {
 	Last60Min int64 `json:"last_60_min"`