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/main.rs

@@ -82,6 +82,18 @@ enum Cmd {
         /// Set via `services.hyperhive.agentMemoryMax`.
         #[arg(long, default_value = "4G")]
         agent_memory_max: String,
+        /// Per-model USD prices (per million tokens) for the hive-wide
+        /// ST4TS cost estimate, as a JSON object mapping a model-family
+        /// short name to `{input, output, cache_read, cache_write}`.
+        /// Keys are matched case-insensitively as a substring of the
+        /// model id; models not covered fall back to the built-in
+        /// estimate. Set via the `services.hyperhive.modelPrices` NixOS
+        /// option.
+        #[arg(
+            long,
+            default_value = r#"{"opus":{"input":15.0,"output":75.0,"cache_read":1.5,"cache_write":18.75},"sonnet":{"input":3.0,"output":15.0,"cache_read":0.3,"cache_write":3.75},"haiku":{"input":0.8,"output":4.0,"cache_read":0.08,"cache_write":1.0}}"#
+        )]
+        model_prices: String,
     },
     /// Spawn a new agent container directly (`hive-agent-<name>`). Bypasses
     /// the approval queue — use only as an operator on the host. For
@@ -152,6 +164,7 @@ async fn main() -> Result<()> {
             context_window_tokens,
             agent_cpu_quota,
             agent_memory_max,
+            model_prices,
         } => {
             cmd_serve(
                 hyperhive_flake,
@@ -163,6 +176,7 @@ async fn main() -> Result<()> {
                 context_window_tokens,
                 agent_cpu_quota,
                 agent_memory_max,
+                model_prices,
                 &cli.socket,
             )
             .await
@@ -221,6 +235,7 @@ async fn cmd_serve(
     context_window_tokens: String,
     agent_cpu_quota: String,
     agent_memory_max: String,
+    model_prices: String,
     socket: &std::path::Path,
 ) -> Result<()> {
     // Move any host-side state still at the legacy flat layout into its
@@ -230,6 +245,8 @@ async fn cmd_serve(
     hive_c0re::paths::relocate_legacy_state();
     let cwt: std::collections::HashMap<String, u64> = serde_json::from_str(&context_window_tokens)
         .context("--context-window-tokens: invalid JSON")?;
+    let prices: hive_c0re::hive_stats::PriceTable =
+        serde_json::from_str(&model_prices).context("--model-prices: invalid JSON")?;
     let coord = Arc::new(Coordinator::open(
         &db,
         hyperhive_flake,
@@ -240,6 +257,7 @@ async fn cmd_serve(
         cwt,
         agent_cpu_quota,
         agent_memory_max,
+        prices,
     )?);
     manager_server::start(coord.clone())?;
     // Idempotent pre-flight: rewrite pre-meta-layout applied