fix(query): wait briefly for dashboard fanout

[smithclay]

Summary

• add a bounded query-admission wait for compatibility query routes so Grafana dashboard fanout can drain through short-lived heavy query slots
• expose CANARDSTACK_QUERY_ADMISSION_WAIT_MS / _SECS and document the default
• update the provisioned dashboard test to match the current OpenTelemetry demo dashboard

Validation

• cargo test
• cargo clippy --all-targets --all-features --locked -- -D warnings
• rebuilt local compose image with docker compose -f compose.yaml -f compose.build.yaml up -d --build canardstack grafana
• replayed six concurrent OTel demo Prometheus range queries: all returned 200
• reloaded Grafana dashboard in browser: No data count 0