Add RigExpert Fobos SDR source

[UR4MCB]

Adds RF-mode support for RigExpert Fobos SDR receivers through the existing SoapySDR / soapy_connector path. The implementation adds a new fobos source type, feature detection for SoapySDR driver=fobos, and Fobos-specific RF controls for clock source, LNA and VGA. HF Direct Sampling is intentionally not included because Fobos direct sampling provides HF1/HF2 real-channel samples, not normal I/Q.

[UR4MCB]

Additional local hardware validation completed.

Test setup:

• RigExpert Fobos SDR, HW revision 3.0.0
• Agile firmware 3.1.0
• SoapyFobosSDR detected as driver=fobos
• soapy_connector 0.7.0-dev
• OpenWebRX v1.3.0-dev
• Ubuntu 26.04

Validated behavior:

• The new fobos source type is detected by OpenWebRX.
• The receiver appears in the OpenWebRX Settings UI as RigExpert Fobos SDR.
• RF-mode profile starts successfully through soapy_connector.
• FM broadcast reception works; audio plays in the browser.
• The Settings UI exposes the Fobos-specific controls:
  • Clock source
  • Fobos LNA
  • Fobos VGA
• Saving settings from the UI works.
• Changing profiles updates frequency through the control socket.
• Changing profile-specific LNA/VGA values is translated correctly to rf_gain, for example:
  • LNA=0,VGA=12
  • LNA=2,VGA=20
• No unknown key errors for fobos_lna / fobos_vga.
• No direct_samp option is sent in this RF-only implementation.
• Clean shutdown confirmed.

Scope note:

HF Direct Sampling is intentionally not enabled in this PR. According to the Fobos SDR API documentation, direct sampling returns interleaved HF1/HF2 real-channel samples rather than normal complex I/Q samples. Therefore, generic OpenWebRX complex-IQ handling is not sufficient for that mode without a dedicated direct-sampling processing path.