⚡️(frontend) backport phone number formatting to the backend

Use the Python port of Google's phone number library on the backend
instead of the JS one on the client. Saves 140Kb of unnecessary JS
and avoids a complex dynamic import that would have been required to
optimize loading.

The transformation is static: the phone number lives in the Django
settings, so the backend can format it once and pass the result to
the client. This avoids every client loading the 140Kb library and
re-computing the same information.

Moreover, within a single client, the transformation was previously
re-computed several times during a webapp lifecycle.

src/backend/core/utils.py

@@ -12,6 +12,7 @@ import mimetypes
 import random
 import secrets
 import string
+from functools import lru_cache
 from typing import List, Optional
 from uuid import uuid4
 
@@ -22,6 +23,7 @@ import aiohttp
 import boto3
 import botocore
 import magic
+import phonenumbers
 from asgiref.sync import async_to_sync
 from livekit.api import (  # pylint: disable=E0611
     AccessToken,
@@ -455,3 +457,58 @@ def generate_upload_policy(file):
     )
 
     return policy
+
+
+@lru_cache(maxsize=1)
+def _format_telephony_phone_number(raw_number, default_country):
+    """Parse a configured phone number and return (country, international_format).
+
+    Returns (None, None) if the inputs are missing or the number cannot be
+    parsed. Logs a warning on parse failure so operators see the misconfiguration.
+    """
+    if not raw_number or not default_country:
+        return None, None
+
+    try:
+        parsed = phonenumbers.parse(raw_number, default_country)
+    except phonenumbers.NumberParseException:
+        logger.warning(
+            "ROOM_TELEPHONY_PHONE_NUMBER %r is not a valid phone number for "
+            "default country %r; telephony block will be returned without "
+            "formatted number.",
+            raw_number,
+            default_country,
+        )
+        return None, None
+
+    country = phonenumbers.region_code_for_number(parsed)
+    international = phonenumbers.format_number(
+        parsed, phonenumbers.PhoneNumberFormat.INTERNATIONAL
+    )
+    return country, international
+
+
+def build_telephony_config():
+    """Build the telephony block of the frontend configuration."""
+    if not settings.ROOM_TELEPHONY_ENABLED:
+        return {"enabled": False}
+
+    country, international = _format_telephony_phone_number(
+        settings.ROOM_TELEPHONY_PHONE_NUMBER,
+        settings.ROOM_TELEPHONY_DEFAULT_COUNTRY,
+    )
+
+    if international is None:
+        logger.warning(
+            "Telephony is enabled but ROOM_TELEPHONY_PHONE_NUMBER %r with "
+            "default country %r could not be formatted; telephony will be disabled.",
+            settings.ROOM_TELEPHONY_PHONE_NUMBER,
+            settings.ROOM_TELEPHONY_DEFAULT_COUNTRY,
+        )
+        return {"enabled": False}
+
+    return {
+        "enabled": True,
+        "default_country": country,
+        "international_phone_number": international,
+    }