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/alert_detector.go

@@ -15,8 +15,6 @@ import (
 
 // AlertDetectorConfig holds configuration for alert detection
 type AlertDetectorConfig struct {
-	Enabled               bool
-	Interval              time.Duration
 	ErrorSpikeThreshold   float64 // Error rate threshold (0.1 = 10%)
 	ErrorSpikeWindow      time.Duration
 	QuotaWarningThreshold float64 // Quota usage threshold (0.9 = 90%)
@@ -27,8 +25,6 @@ type AlertDetectorConfig struct {
 // DefaultAlertDetectorConfig returns default configuration
 func DefaultAlertDetectorConfig() AlertDetectorConfig {
 	return AlertDetectorConfig{
-		Enabled:               true,
-		Interval:              1 * time.Minute,
 		ErrorSpikeThreshold:   0.1, // 10% error rate
 		ErrorSpikeWindow:      5 * time.Minute,
 		QuotaWarningThreshold: 0.9, // 90% quota used
@@ -65,29 +61,12 @@ func NewAlertDetector(db, logDB *gorm.DB, rdb *redis.Client, statsService *servi
 	}
 }
 
-// Start begins the alert detection loop
-func (d *AlertDetector) Start(ctx context.Context) {
-	if d == nil || !d.config.Enabled {
+// RunOnce executes a single detection cycle. Called by scheduler.
+func (d *AlertDetector) RunOnce(ctx context.Context) {
+	if d == nil || d.db == nil {
 		return
 	}
-	if ctx == nil {
-		ctx = context.Background()
-	}
-
-	ticker := time.NewTicker(d.config.Interval)
-	defer ticker.Stop()
-
-	d.logger.Info("alert detector started", "interval", d.config.Interval)
-
-	for {
-		select {
-		case <-ctx.Done():
-			d.logger.Info("alert detector stopped")
-			return
-		case <-ticker.C:
-			d.detectOnce(ctx)
-		}
-	}
+	d.detectOnce(ctx)
 }
 
 // detectOnce runs all detection rules once