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

docs/web-ui/dashboard.md

@@ -333,10 +333,14 @@ the data is fetched on tab activation and on window change. Rendered
 with plain tables + CSS bars — the dashboard bundle ships no chart
 library.
 
-The cost figure is a deliberately rough estimate from an approximate
-per-model price table (`est_cost_usd`); it drifts with list pricing and
-is labelled accordingly. (A follow-up can move the table to a nix
-option so it's operator-tunable.)
+The cost figure is a deliberately rough estimate from a per-model
+price table (`est_cost_usd`); it drifts with list pricing and is
+labelled accordingly. The table is operator-tunable via the
+`services.hyperhive.modelPrices` nix option — each key is a
+model-family short name (matched case-insensitively as a substring of
+the model id, longest match wins) mapping to
+`{ input, output, cache_read, cache_write }` USD-per-million-token
+prices. Models not covered fall back to hive-c0re's built-in estimate.
 
 ## P33RS tab