fix(scheduler): harden scheduled operations with stale cleanup, cron validation, and design fixes (#549)

* fix(scheduler): clean up stale runs on startup and auto-disable invalid cron tasks

- Add markStaleRunsAsFailed() bulk DB method with status index
- Clean up orphaned 'running' records on scheduler startup
- Auto-disable tasks when cron expression becomes invalid at execution time
- Promote CRUD debug logs to standard logs for scheduled task admin actions
- Add diagnostic logging for task pre-checks, action timing, and prune fallback

* refactor(scheduling): extract shared types and fix design system violations

- Extract ScheduledTask, TaskRun, NodeOption to shared types file
- Extract getCronDescription and formatTimestamp to shared utilities
- Fix formatTimestamp falsy-zero null check
- Tighten last_status type to 'success' | 'failure' | null
- Add strokeWidth={1.5} to all action icons per design system
- Add sr-only DialogDescription for accessibility
- Wrap Sheet run history in ScrollArea
- Fix delete button styling to match design system pattern
- Change manual trigger toast from "executed" to "triggered"

* test(scheduler): add tests for snapshot, remote update, stale cleanup, and cron invalidation

- Add stale run cleanup tests (bulk markStaleRunsAsFailed, logging)
- Add cron invalidation test (auto-disable, error message)
- Add executeSnapshot tests (fleet capture, empty stacks)
- Add executeUpdateRemote tests (proxy success, remote error)
- Document stale run cleanup and cron auto-disable in troubleshooting docs

backend/src/services/DatabaseService.ts

@@ -437,6 +437,7 @@ export class DatabaseService {
       );
 
       CREATE INDEX IF NOT EXISTS idx_scheduled_task_runs_task ON scheduled_task_runs(task_id);
+      CREATE INDEX IF NOT EXISTS idx_scheduled_task_runs_status ON scheduled_task_runs(status);
       CREATE INDEX IF NOT EXISTS idx_scheduled_tasks_next_run ON scheduled_tasks(next_run_at);
 
       CREATE TABLE IF NOT EXISTS stack_labels (
@@ -1531,6 +1532,13 @@ export class DatabaseService {
         ).all(taskId) as ScheduledTaskRun[];
     }
 
+    public markStaleRunsAsFailed(): number {
+        const result = this.db.prepare(
+            'UPDATE scheduled_task_runs SET status = ?, completed_at = ?, error = ? WHERE status = ?'
+        ).run('failure', Date.now(), 'Server restarted during execution', 'running');
+        return result.changes;
+    }
+
     public cleanupOldTaskRuns(retentionDays = 30): void {
         const cutoff = Date.now() - (retentionDays * 24 * 60 * 60 * 1000);
         this.db.prepare('DELETE FROM scheduled_task_runs WHERE started_at < ?').run(cutoff);