feat(c0re): /api/stats-hive — hive-wide turn-stats rollup

#1424 P2 backend. New hive_stats module aggregates every agent's
turn-stats.sqlite read-only (reusing Coordinator::kept_state_names +
agent_harness_dir, skipping missing/unreadable dbs) into swarm totals,
a busiest-first per-agent rollup, swarm model mix, and a labelled USD
cost estimate (rough model->price table; can move to a nix option
later). Exposed as GET /api/stats-hive?window=. Dashboard UI follows.

hive-c0re/src/lib.rs

@@ -32,6 +32,7 @@ pub mod events_vacuum;
 pub mod flake_check;
 pub mod forge;
 pub mod gateway_nginx;
+pub mod hive_stats;
 pub mod knowledge;
 pub mod lifecycle;
 pub mod limits;