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/coordinator.rs

@@ -86,6 +86,14 @@ pub struct Coordinator {
     pub agent_cpu_quota: String,
     /// Per-agent systemd `MemoryMax=` value (e.g. `"4G"`). Same drop-in.
     pub agent_memory_max: String,
+    /// Operator-tunable model→price table backing the hive-wide cost
+    /// estimate on the ST4TS tab. Set via `services.hyperhive.modelPrices`
+    /// and passed to `hive-c0re serve --model-prices <json>`. Models not
+    /// in the table fall back to the built-in estimate in `hive_stats`.
+    /// hive-c0re-local (read only by the `/api/stats-hive` handler), so —
+    /// unlike `context_window_tokens` — it is *not* part of `HiveEnv` and
+    /// is never injected into containers.
+    pub model_prices: crate::hive_stats::PriceTable,
     agents: Mutex<HashMap<String, AgentSocket>>,
     /// Agents whose lifecycle action (currently just spawn) is in flight.
     /// Read by the dashboard to render a spinner; cleared when the action
@@ -279,6 +287,7 @@ impl Coordinator {
         context_window_tokens: std::collections::HashMap<String, u64>,
         agent_cpu_quota: String,
         agent_memory_max: String,
+        model_prices: crate::hive_stats::PriceTable,
     ) -> Result<Self> {
         let broker = Broker::open(db_path).context("open broker")?;
         let approvals = Approvals::open(db_path).context("open approvals")?;
@@ -314,6 +323,7 @@ impl Coordinator {
             context_window_tokens,
             agent_cpu_quota,
             agent_memory_max,
+            model_prices,
             agents: Mutex::new(HashMap::new()),
             transient: Mutex::new(HashMap::new()),
             recent_transient: Mutex::new(HashMap::new()),