feat(query-engine): RQE tree query route #883

Part of the RQE frontend tracker. Decisions live in the tracker. Stacked on A3.

What

Query engine (asap-query-engine):

1. engines/simple_engine/rqe.rs: evaluate an RQE tree at a time t:
   • leaf: look up rqe_configs[id] → AggregationIdInfo; timestamps = aligned end (same alignment as PromQL) minus lookback_ms; QueryMetadata from capability/phi/k/grouping; then the existing context tail (build_sql_execution_context_tail-style) and execute_context; apply the config's roll-up if any
   • agg: regroup the child's results by by with avg/min/max/sum
   • binary: combine_vector_vector / combine_scalar
2. Route POST /api/v1/rqe_query with body {"expr": <tree>, "time": <unix secs>}, answering Prometheus vector JSON. Output labels must equal what Prometheus returns for the equivalent PromQL (including __name__ handling), because fidelity scoring matches series by exact label set.
3. No fallback to another backend. No data yet → empty result. Unknown RQE id or invalid tree → HTTP 400.

Done when

• tests per node kind; error/empty cases; a leaf answers the same as the PromQL path for the same plan/config