feat: add 3CX telephony provider with Asterisk ARA provisioning

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>

api/schemas/telephony_config.py

@@ -28,6 +28,10 @@ from api.services.telephony.providers.telnyx.config import (
     TelnyxConfigurationRequest,
     TelnyxConfigurationResponse,
 )
+from api.services.telephony.providers.three_cx.config import (
+    ThreeCxConfigurationRequest,
+    ThreeCxConfigurationResponse,
+)
 from api.services.telephony.providers.twilio.config import (
     TwilioConfigurationRequest,
     TwilioConfigurationResponse,
@@ -50,6 +54,7 @@ TelephonyConfigRequest = Annotated[
         CloudonixConfigurationRequest,
         PlivoConfigurationRequest,
         TelnyxConfigurationRequest,
+        ThreeCxConfigurationRequest,
         TwilioConfigurationRequest,
         VobizConfigurationRequest,
         VonageConfigurationRequest,
@@ -73,6 +78,7 @@ class TelephonyConfigurationResponse(BaseModel):
     cloudonix: Optional[CloudonixConfigurationResponse] = None
     ari: Optional[ARIConfigurationResponse] = None
     telnyx: Optional[TelnyxConfigurationResponse] = None
+    three_cx: Optional[ThreeCxConfigurationResponse] = None
 
 
 # ---------------------------------------------------------------------------
@@ -142,6 +148,8 @@ __all__ = [
     "TelephonyConfigurationResponse",
     "TelnyxConfigurationRequest",
     "TelnyxConfigurationResponse",
+    "ThreeCxConfigurationRequest",
+    "ThreeCxConfigurationResponse",
     "TwilioConfigurationRequest",
     "TwilioConfigurationResponse",
     "VobizConfigurationRequest",