Feature: Configurable input_gain and output_gain for media bridge

[davidcforbes]

Description

WebRTC browsers and PSTN trunks often have significant audio level mismatches. PSTN audio is too quiet for browser playback, while Chrome's AGC over-amplifies browser audio causing clipping on the PSTN side.

Implementation

Configurable per-leg gain multipliers:

• input_gain — boosts Leg B (PSTN) audio for browser playback
• output_gain — attenuates Leg A (browser) audio to prevent PSTN clipping
• Applied during transcoding via PCM-level gain with soft clipping
• Also applied to recorder for balanced recording levels

Config

[proxy.recording]
input_gain = 2.0    # Boost PSTN audio
output_gain = 0.7   # Reduce browser audio

Files

• src/config.rs — input_gain/output_gain on RecordingPolicy
• src/media/mod.rs — apply_gain() utility
• src/media/transcoder.rs — with_gain() builder method
• src/proxy/proxy_call/media_bridge.rs — gain plumbing to forward_track

Reference: davidcforbes/rustpbx@3c7567d, davidcforbes/rustpbx@0a7b194