feat: async method parity, prefork logging, streaming fixes

[pratyush618]

Summary

• 18 missing async methods — aget_job, alist_jobs, alist_jobs_filtered, ajob_dag, ametrics_timeseries, ajob_errors, atask_logs, aquery_logs, acancel_running_job, aenqueue_many, apurge_completed, apurge_dead, arevoke_task, areplay_history, aarchive, alist_archived, apause/aresume, apaused_queues
• Prefork child logging — logging.getLogger("taskito.prefork.child") with structured log calls for ready, execution, errors, shutdown, teardown failures
• to_dict() namespace — adds missing namespace field to JobResult.to_dict()
• Streaming error logging — stream()/astream() now log warnings instead of silently yielding raw strings on deserialization failure

Test plan

• 357 pass, 9 skipped
• ruff clean, mypy 0 errors (118 files)