[refactor](trx-backend-soapysdr): remove feature gating, require real hardware

Drop optional feature gating - SoapySDR hardware support is now required.
Make soapysdr a required dependency in Cargo.toml instead of optional.
Update server to always enable soapysdr backend and its dependencies.
Simplify initialization to always use RealIqSource instead of conditional
fallback to MockIqSource.

This assumes SoapySDR library is installed on the system.

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

src/trx-server/Cargo.toml

@@ -11,7 +11,6 @@ build = "build.rs"
 [features]
 default = ["soapysdr"]
 soapysdr = ["trx-backend/soapysdr"]
-soapysdr-sys = ["soapysdr", "trx-backend/soapysdr-sys"]
 
 [dependencies]
 tokio = { workspace = true, features = ["full"] }
@@ -27,8 +26,8 @@ bytes = "1"
 cpal = "0.15"
 opus = "0.3"
 trx-app = { path = "../trx-app" }
-trx-backend = { path = "trx-backend" }
-trx-backend-soapysdr = { path = "trx-backend/trx-backend-soapysdr", optional = true }
+trx-backend = { path = "trx-backend", features = ["soapysdr"] }
+trx-backend-soapysdr = { path = "trx-backend/trx-backend-soapysdr" }
 trx-core = { path = "../trx-core" }
 trx-aprs = { path = "../decoders/trx-aprs" }
 trx-cw = { path = "../decoders/trx-cw" }