[refactor](trx-server): access soapysdr through trx-backend facade

Remove direct dependency on trx-backend-soapysdr from server Cargo.toml.
Re-export SoapySdrRig from trx-backend instead so server accesses it
through the proper facade. This keeps sub-backends internal to trx-backend
and maintains proper module organization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/trx-server/trx-backend/src/lib.rs

@@ -13,6 +13,8 @@ mod dummy;
 use trx_backend_ft450d::Ft450d;
 #[cfg(feature = "ft817")]
 use trx_backend_ft817::Ft817;
+#[cfg(feature = "soapysdr")]
+pub use trx_backend_soapysdr::SoapySdrRig;
 
 /// Connection details for instantiating a rig backend.
 #[derive(Debug, Clone)]