[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>

Cargo.lock

@@ -2525,7 +2525,6 @@ dependencies = [
  "trx-app",
  "trx-aprs",
  "trx-backend",
- "trx-backend-soapysdr",
  "trx-core",
  "trx-cw",
  "trx-decode-log",