feat(stats): make ST4TS model price table operator-tunable

The hive-wide cost estimate on the dashboard's ST4TS tab used a
hard-coded model->price table in hive_stats.rs. Anthropic list pricing
drifts, so move the table to a nix option operators can keep current
without a code change.

- New `services.hyperhive.modelPrices` option: attrset of model-family
  short name -> { input, output, cache_read, cache_write } USD per
  million tokens. Passed to `hive-c0re serve --model-prices <json>`.
- hive_stats: `Prices` is now public + Deserialize; add `PriceTable`
  type and `resolve_prices` (longest case-insensitive substring key
  wins) with the old hard-coded table preserved as `builtin_prices`
  fallback for any model not covered.
- Coordinator holds the parsed table (hive-c0re-local, not injected
  into containers, so not part of HiveEnv); `/api/stats-hive` reads it.
- Docs: dashboard.md ST4TS cost note updated; option self-documents
  via nixosOptionsDoc.

Closes #1434

hive-c0re/src/dashboard.rs

@@ -1791,9 +1791,16 @@ struct StatsHiveQuery {
 /// Hive-wide turn-stats rollup for the dashboard swarm-stats view.
 /// Aggregates every agent's `hyperhive-turn-stats.sqlite` read-only
 /// (skips missing/unreadable ones). Window defaults to `24h`.
-async fn api_stats_hive(axum::extract::Query(q): axum::extract::Query<StatsHiveQuery>) -> Response {
+async fn api_stats_hive(
+    State(state): State<AppState>,
+    axum::extract::Query(q): axum::extract::Query<StatsHiveQuery>,
+) -> Response {
     let window = crate::hive_stats::Window::parse(q.window.as_deref().unwrap_or("24h"));
-    axum::Json(crate::hive_stats::hive_snapshot(window)).into_response()
+    axum::Json(crate::hive_stats::hive_snapshot(
+        window,
+        &state.coord.model_prices,
+    ))
+    .into_response()
 }
 
 /// Live per-agent-container CPU + memory load from cgroup v2. Samples