feat: Add Supervisor Status API endpoints (Phase 3 Task 3.5)

Implement REST API endpoints for querying supervisor status:

- GET /api/v1/contracts/{id}/supervisor/status
  Returns current supervisor status including task_id, state, phase,
  current_activity, progress, last_heartbeat, and pending_task_ids

- GET /api/v1/contracts/{id}/supervisor/heartbeats?limit=10
  Returns paginated supervisor activity history from history_events

- POST /api/v1/contracts/{id}/supervisor/sync
  Triggers a sync to refresh the supervisor's last_activity timestamp

New types added:
- SupervisorStatusResponse - Status endpoint response
- SupervisorHeartbeatEntry - Individual heartbeat history entry
- SupervisorHeartbeatHistoryResponse - Heartbeat history with pagination
- SupervisorSyncResponse - Sync endpoint response
- HeartbeatHistoryQuery - Query params for heartbeats endpoint

Repository helpers:
- get_supervisor_status() - Combined info from supervisor_states and tasks
- get_supervisor_activity_history() - Activity timeline from history_events
- count_supervisor_activity_history() - Total count for pagination
- sync_supervisor_state() - Refresh last_activity timestamp

Error handling:
- 404 for contract not found (CONTRACT_NOT_FOUND)
- 404 for no supervisor (SUPERVISOR_NOT_FOUND)
- Proper fallback when supervisor_state record doesn't exist but task does

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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