refactor(cron): migrate cron jobs to foundation scheduler

Replace custom goroutine-based scheduling in cron jobs with centralized
foundation scheduler. Each cron job now exposes a RunOnce method called
by the scheduler instead of managing its own ticker loop.

Changes:
- Remove interval/enabled config from cron job structs
- Convert Start() methods to RunOnce() for all cron jobs
- Add scheduler setup in main.go with configurable intervals
- Update foundation dependency to v0.6.0 for scheduler support
- Update tests to validate RunOnce nil-safety

internal/cron/quota_reset.go

@@ -12,37 +12,21 @@ import (
 )
 
 type QuotaResetter struct {
-	db       *gorm.DB
-	sync     *service.SyncService
-	interval time.Duration
+	db   *gorm.DB
+	sync *service.SyncService
 }
 
-func NewQuotaResetter(db *gorm.DB, sync *service.SyncService, interval time.Duration) *QuotaResetter {
-	if interval <= 0 {
-		interval = 5 * time.Minute
-	}
-	return &QuotaResetter{db: db, sync: sync, interval: interval}
+func NewQuotaResetter(db *gorm.DB, sync *service.SyncService) *QuotaResetter {
+	return &QuotaResetter{db: db, sync: sync}
 }
 
-func (q *QuotaResetter) Start(ctx context.Context) {
+// RunOnce executes a single quota reset check. Called by scheduler.
+func (q *QuotaResetter) RunOnce(ctx context.Context) {
 	if q == nil || q.db == nil {
 		return
 	}
-	if ctx == nil {
-		ctx = context.Background()
-	}
-	ticker := time.NewTicker(q.interval)
-	defer ticker.Stop()
-
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-ticker.C:
-			if err := q.resetOnce(ctx); err != nil {
-				slog.Default().Warn("quota reset failed", "err", err)
-			}
-		}
+	if err := q.resetOnce(ctx); err != nil {
+		slog.Default().Warn("quota reset failed", "err", err)
 	}
 }