Add SendBlue iMessage integration with admin-only access

- New imessage blueprint: webhook receives inbound iMessages, runs through
  LangChain agent, replies via SendBlue REST API
- Admin-only: only users with lldap_admin group can use iMessage channel
- Admin endpoints to link/unlink imessage_number on user accounts
- Add imessage_number field to User model (needs aerich migration)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blueprints/users/models.py

@@ -10,11 +10,18 @@ class User(Model):
     username = fields.CharField(max_length=255)
     password = fields.BinaryField(null=True)  # Hashed - nullable for OIDC users
     email = fields.CharField(max_length=100, unique=True)
-    whatsapp_number = fields.CharField(max_length=30, unique=True, null=True, index=True)
+    whatsapp_number = fields.CharField(
+        max_length=30, unique=True, null=True, index=True
+    )
+    imessage_number = fields.CharField(
+        max_length=30, unique=True, null=True, index=True
+    )
 
     # Email channel fields
     email_enabled = fields.BooleanField(default=False)
-    email_hmac_token = fields.CharField(max_length=16, unique=True, null=True, index=True)
+    email_hmac_token = fields.CharField(
+        max_length=16, unique=True, null=True, index=True
+    )
 
     # OIDC fields
     oidc_subject = fields.CharField(