Implement thread-bound scheduled tasks

[shuxueshuxue]

Summary

• add a new thread-bound scheduled_tasks backend domain with scheduler/runtime, run tracking, queue metadata propagation, and panel APIs
• wire the frontend tasks area to scheduled-tasks APIs, add a scheduled task editor, and add a direct /tasks?tab=scheduled navigation entry
• add regression coverage for scheduled task storage/runtime behavior, dependency requirements, and frontend scheduled task navigation/state

Test Plan

• uv run pytest -q tests/test_project_dependencies.py tests/test_scheduled_task_service.py tests/test_scheduled_task_api.py tests/test_followup_requeue.py
• cd frontend/app && npx vitest run src/lib/tasks-navigation.test.ts src/store/app-store.scheduled-tasks.test.ts
• cd frontend/app && npx tsc -p tsconfig.app.json --noEmit && npm run build
• isolated manual proof: start backend with temporary LEON_DB_PATH, create a scheduled task bound to a new thread, trigger /api/panel/scheduled-tasks/{id}/run, and verify assistant history contains PR_SCHEDULED_AGENT_PROOF