hive-c0re: drop utoipa-swagger-ui, serve openapi.json directly

Swagger UI itself is nginx-hosted now (iris's 86a39c4c), so c0re
carrying its own vendored copy via utoipa-swagger-ui was a straight
duplicate — dropped the dependency (root Cargo.toml + hive-c0re's),
swapped the SwaggerUi::new(...).url(...) mount for a plain
/api/openapi.json GET route serving the same OpenApi doc as JSON.

Verified: cargo build/clippy/test -p hive-c0re clean, Cargo.lock
dropped utoipa-swagger-ui + utoipa-swagger-ui-vendored with no other
changes, nix fmt clean.

hive-c0re/src/dashboard/mod.rs

@@ -7,18 +7,23 @@ use std::sync::Arc;
 
 use anyhow::{Context, Result};
 use axum::{
+    Json,
     http::StatusCode,
     response::{IntoResponse, Response},
+    routing::get,
 };
 use utoipa::OpenApi;
 use utoipa_axum::{router::OpenApiRouter, routes};
-use utoipa_swagger_ui::SwaggerUi;
 
 use crate::coordinator::Coordinator;
 use crate::lifecycle;
 
-/// Root of the auto-generated `OpenAPI` spec (`/api/openapi.json`, browsable
-/// at `/api/docs`) — see [`utoipa`]. Only routes carrying a
+/// Root of the auto-generated `OpenAPI` spec, served raw at
+/// `/api/openapi.json` — see [`utoipa`]. Swagger UI itself (browsable at
+/// `/api/docs`) is nginx-hosted straight from the nix store now (see
+/// `nix/host-modules/hive-gateway/vhosts.nix`'s `swaggerUiLocations`);
+/// c0re only needs to serve the JSON this route generates, not the whole
+/// vendored UI. Only routes carrying a
 /// `#[utoipa::path(...)]` annotation show up; the rest of the (much
 /// larger) route table below is undocumented for now. Deliberately
 /// incremental: an unannotated route just doesn't appear in the spec,
@@ -206,8 +211,15 @@ pub async fn serve(
         .routes(routes!(state_snapshot::dashboard_stream))
         .routes(routes!(state_snapshot::dashboard_history))
         .split_for_parts();
+    // Just the JSON, not the UI — Swagger UI itself is nginx-hosted from
+    // the nix store (see the module doc comment above `ApiDoc`). `api`
+    // is `Clone`; each request gets its own owned copy for `Json` to
+    // serialize.
     let app = router
-        .merge(SwaggerUi::new("/api/docs").url("/api/openapi.json", api))
+        .route(
+            "/api/openapi.json",
+            get(move || async move { Json(api.clone()) }),
+        )
         .with_state(AppState {
             coord,
             webhook_secret,