add account preferences model and API endpoints for user settings

backend/authentication/models.py

@@ -113,6 +113,23 @@ class ToolshedUser(AbstractUser):
         return self.public_identity.public_key
 
 
+class AccountPreference(models.Model):
+    """A single account-level (server-synced, cross-device) user preference, stored as a key/value pair.
+
+    Device-level preferences are intentionally *not* stored here - they stay in the browser's
+    local storage since they describe the device, not the account.
+    """
+    user = models.ForeignKey(ToolshedUser, on_delete=models.CASCADE, related_name='preferences')
+    key = models.CharField(max_length=255)
+    value = models.JSONField()
+
+    class Meta:
+        unique_together = ('user', 'key')
+
+    def __str__(self):
+        return f"{self.user}: {self.key}"
+
+
 class FriendRequestOutgoing(models.Model):
     secret = models.CharField(max_length=255)
     befriender_user = models.ForeignKey(ToolshedUser, on_delete=models.CASCADE, related_name='friend_requests_outgoing')