#818 Workflows list perf: dashboard fetches + client-groups ALL runs (9845); add grouped-summary endpoint (#agentic)

The dashboard workflows page fetches every run (days-windowed, ~9845 for a busy tenant) with no working pagination (backend ignores limit/offset; days overrides pagination) and groups them client-side, re-sorting/re-fetching on a 2.5-5s poll. Fix: new GET /api/v1/workflows/summary returns one row per definition_hash with total_runs, status_counts, latest_created_at, and latest N runs (default 10) + metadata totals - so the client renders the grouped list from ~hundreds of small rows. Backend done; dashboard consumer next.