#773 Activity logs API broken: ACTIVITY_LOG_NOT_FOUND + query_shell_output signature crash

Two bugs in /api/v1/logs activity endpoints. (1) Activity logs never persisted: enterprise activity worker used core engine.tools.activity_logger stub (Python-logger only); query_activity_logs_by_id stub returns [] -> ACTIVITY_LOG_NOT_FOUND. Fixed: added activity_logs DataShard table (writer+queries) in enterprise/tools/datashard_logger.py mirroring task_logs; worker now persists each start/complete/fail event; logs API reads enterprise queries. (2) query_shell_output core stub had signature (tenant,wf,task_id,stream,limit) + returned dict, but logs API calls it with activity_id/offset/sequence_gt and iterates result as a list -> TypeError 500. Fixed: stub now accepts full signature and returns [] (graceful empty). Surfaced once durable leaves actually run as activities (#771).