One-shot delayed execution: extend scheduler with single-fire tasks

[kovtcharov]

Summary

Extend the task scheduler (#550) to support one-shot delayed execution — tasks that fire once after a delay and then auto-purge. Used by the autonomous loop to schedule follow-ups.

Parent issue: #555 (Autonomous mode)

New Interval Types

• "once_after_30m" — run once after 30 minutes
• "once_after_2h" — run once after 2 hours
• "once_at_9am" — run once at a specific time
• "once_at_2026-03-20T14:00" — run once at exact datetime

Behavior

• One-shot tasks execute exactly once, then set status to completed
• Completed one-shots auto-purge from scheduled_tasks table after 24h
• Multiple one-shots respect the 5-min minimum cool-down between executions
• One-shots appear in the schedule list with a "once" badge

Implementation

Extend parse_interval() in scheduler.py to recognize once_after_* and once_at_* patterns. After execution, mark as completed instead of rescheduling.

Add purge job: on scheduler startup and every hour, delete completed one-shots older than 24h.

Dependencies

• Scheduled Autonomy: recurring task scheduler for Agent UI #550 — Scheduled Autonomy (base scheduler)

Acceptance Criteria

• once_after_* intervals parsed correctly
• once_at_* intervals parsed correctly (time and datetime)
• Task fires exactly once then marks as completed
• Auto-purge of completed one-shots after 24h
• 5-min cool-down enforced between consecutive one-shot executions
• One-shots visible in schedule list with appropriate badge
• Unit tests for interval parsing
• Integration test: create one-shot, wait, verify it fired and was purged