Api Dashboard Breakdown Retrieve

GET/POST dashboard/breakdown/?breakdown_by=<dimension> -- top-N breakdown of LLM metrics grouped by one dimension. Single parameterized replacement for the legacy per-dimension dashboard/top-<x>/ endpoints (top-keys, top-models, top-providers, top-prompts, top-deployments, top-users), which were retired once the FE moved to ?breakdown_by=<dimension> (respan-frontend #2409). See BE_conventions/urls.md "Dashboard breakdown (parameterized exception)". Args (query params): breakdown_by, sort_by, summary_type, date, timezone_offset, is_test, all_envs Args (POST body): filters Response: {<sort_by_metric>: [{<dimension>, name, <metric_value>, ...}]}