Add daily usage metrics (CPU-minutes, RAM GB-minutes)

Introduce pre-computed daily usage rollups from sandbox_metrics_snapshots.
An hourly background worker aggregates completed days, while today's
usage is computed live from snapshots at query time for freshness.

Backend: new daily_usage table, rollup worker, UsageService, and
GET /v1/capsules/usage endpoint with date range filtering (up to 92 days).

Frontend: replace Usage page placeholder with bar charts (Chart.js),
summary total cards, and preset/custom date range controls.

internal/api/usage_rollup.go

@@ -0,0 +1,84 @@
+package api
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.omukk.dev/wrenn/wrenn/pkg/db"
+)
+
+// DailyUsageRollup pre-computes daily CPU-minute and RAM-MB-minute totals
+// from sandbox_metrics_snapshots. It runs on startup and then every interval.
+type DailyUsageRollup struct {
+	db       *db.Queries
+	interval time.Duration
+}
+
+// NewDailyUsageRollup creates a DailyUsageRollup.
+func NewDailyUsageRollup(queries *db.Queries, interval time.Duration) *DailyUsageRollup {
+	return &DailyUsageRollup{db: queries, interval: interval}
+}
+
+// Start runs the rollup loop until the context is cancelled.
+func (r *DailyUsageRollup) Start(ctx context.Context) {
+	go func() {
+		ticker := time.NewTicker(r.interval)
+		defer ticker.Stop()
+
+		// Run immediately on startup.
+		r.run(ctx)
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				r.run(ctx)
+			}
+		}
+	}()
+}
+
+func (r *DailyUsageRollup) run(ctx context.Context) {
+	teams, err := r.db.GetTeamsWithSnapshots(ctx)
+	if err != nil {
+		slog.Warn("usage rollup: failed to get teams", "error", err)
+		return
+	}
+
+	now := time.Now().UTC()
+	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
+	yesterday := today.AddDate(0, 0, -1)
+
+	for _, teamID := range teams {
+		// Only roll up yesterday (fully completed day). Today's usage is
+		// computed live at query time by UsageService.
+		if err := r.rollupDay(ctx, teamID, yesterday); err != nil {
+			slog.Warn("usage rollup: failed", "team_id", teamID, "day", yesterday.Format("2006-01-02"), "error", err)
+		}
+	}
+}
+
+func (r *DailyUsageRollup) rollupDay(ctx context.Context, teamID pgtype.UUID, day time.Time) error {
+	dayStart := day
+	dayEnd := day.Add(24 * time.Hour)
+
+	row, err := r.db.ComputeDailyUsageForDay(ctx, db.ComputeDailyUsageForDayParams{
+		TeamID:      teamID,
+		SampledAt:   pgtype.Timestamptz{Time: dayStart, Valid: true},
+		SampledAt_2: pgtype.Timestamptz{Time: dayEnd, Valid: true},
+	})
+	if err != nil {
+		return err
+	}
+
+	return r.db.UpsertDailyUsage(ctx, db.UpsertDailyUsageParams{
+		TeamID:       teamID,
+		Day:          pgtype.Date{Time: day, Valid: true},
+		CpuMinutes:   row.CpuMinutes,
+		RamMbMinutes: row.RamMbMinutes,
+	})
+}