Get Execution Query Profile (explain-on-execute read path)

Return the query profile for a PAST execution, by execution_id — the same plan that explain=true returns inline at execute time, read back later. Contains per-stage timings + input/output counts, MVS execution stats, the optimizer summary, and (as it is wired) the shard ExecutionTrace (chosen legs, fusion, push-downs, nprobe, served/shadow planner). Reads the query_profile persisted on the RETRIEVER_EXECUTION analytics event (latest row wins). Returns 404 if the execution has no captured profile. This is the queryable, Studio-friendly counterpart to inline explain.