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/log_cleaner_test.go

@@ -34,7 +34,7 @@ func TestLogCleanerRetentionDeletesOld(t *testing.T) {
 		t.Fatalf("create fresh: %v", err)
 	}
 
-	cleaner := NewLogCleaner(db, nil, 1, 0, time.Minute, nil)
+	cleaner := NewLogCleaner(db, nil, 1, 0, nil)
 	if err := cleaner.cleanOnce(context.Background()); err != nil {
 		t.Fatalf("clean once: %v", err)
 	}
@@ -64,7 +64,7 @@ func TestLogCleanerMaxRecordsKeepsLatest(t *testing.T) {
 		}
 	}
 
-	cleaner := NewLogCleaner(db, nil, 0, 3, time.Minute, nil)
+	cleaner := NewLogCleaner(db, nil, 0, 3, nil)
 	if err := cleaner.cleanOnce(context.Background()); err != nil {
 		t.Fatalf("clean once: %v", err)
 	}
@@ -85,3 +85,15 @@ func TestLogCleanerMaxRecordsKeepsLatest(t *testing.T) {
 		t.Fatalf("expected min id >= 3, got %d", min.ID)
 	}
 }
+
+func TestLogCleanerRunOnceNilSafe(t *testing.T) {
+	// Test nil cleaner
+	var nilCleaner *LogCleaner
+	nilCleaner.RunOnce(context.Background())
+
+	// Test cleaner with nil db
+	cleaner := &LogCleaner{}
+	cleaner.RunOnce(context.Background())
+
+	// Should not panic
+}