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/Cargo.toml

@@ -53,7 +53,6 @@ tracing-subscriber.workspace = true
 problem_details = { version = "0.9.0", features = ["axum"] }
 utoipa.workspace = true
 utoipa-axum.workspace = true
-utoipa-swagger-ui.workspace = true
 
 [dev-dependencies]
 tempfile = "3"