✨(backend) implement recording expiration mechanism

Add expiration system for recordings.

Include option for users to set recordings as permanent (no expiration)
which is the default behavior.

System only calculates expiration dates and tracks status - actual deletion
is handled by Minio bucket lifecycle policies, not by application code.

src/backend/core/models.py

@@ -3,8 +3,9 @@ Declare and configure the models for the Meet core application
 """
 
 import uuid
+from datetime import datetime, timedelta
 from logging import getLogger
-from typing import List
+from typing import List, Optional
 
 from django.conf import settings
 from django.contrib.auth import models as auth_models
@@ -12,6 +13,7 @@ from django.contrib.auth.base_user import AbstractBaseUser
 from django.core import mail, validators
 from django.core.exceptions import PermissionDenied, ValidationError
 from django.db import models
+from django.utils import timezone
 from django.utils.functional import lazy
 from django.utils.text import capfirst, slugify
 from django.utils.translation import gettext_lazy as _
@@ -601,6 +603,34 @@ class Recording(BaseModel):
 
         return f"{settings.RECORDING_OUTPUT_FOLDER}/{self.id}.{self.extension}"
 
+    @property
+    def expired_at(self) -> Optional[datetime]:
+        """
+        Calculate the expiration date based on created_at and RECORDING_EXPIRATION_DAYS.
+        Returns None if no expiration is configured.
+
+        Note: This is a naive and imperfect implementation since recordings are actually
+        saved to the bucket after created_at timestamp is set. The actual expiration
+        will be determined by the bucket lifecycle policy which operates on the object's
+        timestamp in the storage system, not this value.
+        """
+
+        if not settings.RECORDING_EXPIRATION_DAYS:
+            return None
+
+        return self.created_at + timedelta(days=settings.RECORDING_EXPIRATION_DAYS)
+
+    @property
+    def is_expired(self) -> bool:
+        """
+        Determine if the recording has expired by comparing expired_at with current UTC time.
+        Returns False if no expiration is configured or if expiration date is in the future.
+        """
+        if not self.expired_at:
+            return False
+
+        return self.expired_at < timezone.now()
+
 
 class RecordingAccess(BaseAccess):
     """Relation model to give access to a recording for a user or a team with a role."""