Registers a new `three_cx` provider that fronts a 3CX cloud PBX through
an intermediate Asterisk bridge. Save-time hook writes the matching
PJSIP endpoint/aor/auth/registration and dialplan rows to the Asterisk
Realtime Architecture Postgres (via `ASTERISK_ARA_DSN`), so a config
change in the Dograh UI is immediately picked up by Asterisk without a
`pjsip reload`. Strip prefix is honoured at the dialplan layer.
Inbound calls are matched back to a configuration by the dialled
extension (`account_id_credential_field="extension"`), allowing one
shared Asterisk to serve multiple Dograh orgs without collision.
Touches `providers/__init__.py` and `schemas/telephony_config.py` only
— per `providers/AGENTS.md`. Provider/transport/strategies are
duplicated from `ari/` rather than imported, in line with the
cross-provider-import prohibition. See `docs/providers/three_cx.md` for
the Asterisk ARA setup runbook.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
