feat: stats activity-over-time granularity selector (timeline / day / week / month / year)

Backend adds cyclical distributions ByHour (hour-of-day), ByDOW (weekday), ByDOM
(day-of-month) and ByMonthYr (calendar month), all UTC. The Activity-over-time
card gains a selector: Timeline keeps the chronological month trend; Day/Week/
Month/Year show the hour/weekday/day/month distribution as bars.

internal/qso/stats.go

@@ -163,6 +163,13 @@ type Stats struct {
 	ByYear      []Bucket `json:"by_year"`  // chronological
 	ByMonth     []Bucket `json:"by_month"` // "YYYY-MM", chronological
 
+	// Cyclical activity distributions (UTC), for the "activity over time" chart's
+	// day/week/month/year granularity selector.
+	ByHour    []Bucket `json:"by_hour"`     // hour of day 00..23
+	ByDOW     []Bucket `json:"by_dow"`      // day of week Mon..Sun
+	ByDOM     []Bucket `json:"by_dom"`      // day of month 1..31
+	ByMonthYr []Bucket `json:"by_month_yr"` // month Jan..Dec
+
 	// ── Period / contest metrics ──
 	// Meaningful only over a WINDOW: "12 QSO/h" across seventeen years says
 	// nothing, but across a contest weekend it is the score. The window is the
@@ -469,11 +476,43 @@ func (r *Repo) Stats(ctx context.Context, from, to time.Time, contestID string,
 	s.ByMonth = fillMonths(monthC, first, last)
 
 	s.periodMetrics(times, from, to, first, last)
+	s.timeDistributions(times)
 	s.ensureNonNil()
 
 	return s, nil
 }
 
+// timeDistributions builds the cyclical activity buckets (hour-of-day, day-of-week,
+// day-of-month, month-of-year) from the dated QSOs, in UTC. Each is a fixed-length
+// series so the chart axis is stable even for empty slots.
+func (s *Stats) timeDistributions(times []entry) {
+	hour := make([]int, 24)
+	dow := make([]int, 7) // 0 = Monday
+	dom := make([]int, 31)
+	moy := make([]int, 12)
+	for _, e := range times {
+		t := e.t.UTC()
+		hour[t.Hour()]++
+		dow[(int(t.Weekday())+6)%7]++ // shift Sunday(0) → 6 so Monday is first
+		if d := t.Day(); d >= 1 && d <= 31 {
+			dom[d-1]++
+		}
+		moy[int(t.Month())-1]++
+	}
+	for h := 0; h < 24; h++ {
+		s.ByHour = append(s.ByHour, Bucket{Key: fmt.Sprintf("%02d", h), Count: hour[h]})
+	}
+	for i, n := range []string{"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"} {
+		s.ByDOW = append(s.ByDOW, Bucket{Key: n, Count: dow[i]})
+	}
+	for d := 1; d <= 31; d++ {
+		s.ByDOM = append(s.ByDOM, Bucket{Key: fmt.Sprintf("%d", d), Count: dom[d-1]})
+	}
+	for i, n := range []string{"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"} {
+		s.ByMonthYr = append(s.ByMonthYr, Bucket{Key: n, Count: moy[i]})
+	}
+}
+
 // ensureNonNil replaces every nil slice with an empty one.
 //
 // This is NOT cosmetic. A nil Go slice marshals to JSON `null`, not `[]`, and the