feat(v3): human-in-the-loop — interactive steps, pluggable step_type registry
- New states: waiting (step/task), rejected (step)
- New tables: pipeline_human_tasks, pipeline_step_types
- New module: step_registry.py — pluggable step_type metadata
- New module: human.py — human_complete, approval_approve, approval_reject
- Executor: detects interactive step_types, creates human_tasks, enters waiting
- Reject with rollback: approval_reject(rollback_to=step) resets steps and re-runs
- API: human_task_complete, approval_approve, approval_reject, human_task_list
- API: pipeline_step_types, pipeline_register_step_type, pipeline_unregister_step_type
- Built-in interactive types: human_task, approval_gate
- Updated DDL and appcodes
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