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_test.go

@@ -301,30 +301,16 @@ func TestAlertDetectorDetectOnceNilSafe(t *testing.T) {
 	// Should not panic
 }
 
-func TestAlertDetectorStartDisabled(t *testing.T) {
-	db := setupTestDB(t)
+func TestAlertDetectorRunOnceNilSafe(t *testing.T) {
+	// Test nil detector
+	var nilDetector *AlertDetector
+	nilDetector.RunOnce(context.Background())
 
-	config := DefaultAlertDetectorConfig()
-	config.Enabled = false
+	// Test detector with nil db
+	detector := &AlertDetector{}
+	detector.RunOnce(context.Background())
 
-	detector := NewAlertDetector(db, db, nil, nil, config, nil)
-
-	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
-	defer cancel()
-
-	// Should return immediately without blocking
-	done := make(chan struct{})
-	go func() {
-		detector.Start(ctx)
-		close(done)
-	}()
-
-	select {
-	case <-done:
-		// Expected: Start returned immediately because Enabled=false
-	case <-time.After(200 * time.Millisecond):
-		t.Error("Start did not return immediately when disabled")
-	}
+	// Should not panic
 }
 
 func TestDetectMasterMinuteSpikesRPM(t *testing.T) {