Feature: Trunk registration module for inbound PSTN call delivery

[davidcforbes]

Description

Some SIP trunk providers require the PBX to send SIP REGISTER requests to enable inbound call delivery. Without registration, the provider doesn't know where to route DID calls.

Implementation

A new TrunkRegistrationModule that:

• Reads register = true and register_expires from each trunk's config
• Sends SIP REGISTER requests to trunk providers with credential authentication
• Maintains registration with automatic re-registration at 85% of expiry interval
• Implements exponential backoff on failures

Config

[[proxy.trunks]]
name = "my-trunk"
host = "sip.provider.com"
username = "user"
password = "pass"
register = true
register_expires = 300

Files

• src/proxy/trunk_register.rs — new module
• src/proxy/routing/mod.rs — register and register_expires fields on TrunkConfig
• src/app.rs — module registration

Reference: davidcforbes/rustpbx@a1eea50