Gate self-serve registration behind ENABLE_ACCOUNT_REGISTRATION (#25)

## Summary
- Add `ENABLE_ACCOUNT_REGISTRATION` (default `false`) for self-serve sign-up, set via env / control-node secret (`chat_backend_<env>.env`)
- Harden `POST /api/user/create/` to create company + user, return JWTs, and reject when the flag is off
- Expose `GET /api/public/settings/` with `enable_account_registration` for the SPA

Supports frontend: [chat_web_app#31](ai_ml_operations/chat_web_app#31)

## Test plan
- [ ] `cd llm_be && uv run python manage.py test chat_backend.tests.test_views_users`
- [ ] Confirm `POST /api/user/create/` returns 403 when flag unset/false
- [ ] With `ENABLE_ACCOUNT_REGISTRATION=true`, register returns access/refresh + creates company
- [ ] `GET /api/public/settings/` returns the flag
- [ ] Add `ENABLE_ACCOUNT_REGISTRATION=false` to control-node `chat_backend_prod.env` / `chat_backend_beta.env` before deploy (see `.env.prod.example`)Reviewed-on: #25

llm_be/chat_backend/serializers.py

@@ -54,13 +54,55 @@ class CustomUserSerializer(serializers.ModelSerializer):
         fields = "__all__"
         extra_kwargs = {"password": {"write_only": True}}
 
-    # def create(self, validated_data):
-    #     password = validated_data.pop('password',None)
-    #     instance = self.Meta.model(**validated_data)
-    #     if password is not None:
-    #         instance.set_password(password)
-    #     instance.save()
-    #     return instance
+
+class SelfServeRegistrationSerializer(serializers.Serializer):
+    """Minimal payload for public self-serve sign-up (gated by settings)."""
+
+    email = serializers.EmailField(required=True)
+    password = serializers.CharField(min_length=8, write_only=True)
+    first_name = serializers.CharField(
+        required=False, allow_blank=True, max_length=150, default=""
+    )
+    last_name = serializers.CharField(
+        required=False, allow_blank=True, max_length=150, default=""
+    )
+    company_name = serializers.CharField(
+        required=False, allow_blank=True, max_length=256, default=""
+    )
+
+    def validate_email(self, value: str) -> str:
+        email = value.strip().lower()
+        if CustomUser.objects.filter(email__iexact=email).exists():
+            raise serializers.ValidationError("A user with this email already exists.")
+        if CustomUser.objects.filter(username__iexact=email).exists():
+            raise serializers.ValidationError("A user with this email already exists.")
+        return email
+
+    def create(self, validated_data):
+        email = validated_data["email"]
+        password = validated_data["password"]
+        first_name = (validated_data.get("first_name") or "").strip()
+        last_name = (validated_data.get("last_name") or "").strip()
+        company_name = (validated_data.get("company_name") or "").strip()
+        if not company_name:
+            company_name = f"{email}'s workspace"
+
+        company = Company.objects.create(
+            name=company_name,
+            state="NA",
+            zipcode="00000",
+            address="N/A",
+        )
+        user = CustomUser.objects.create_user(
+            username=email,
+            email=email,
+            password=password,
+            first_name=first_name,
+            last_name=last_name,
+            company=company,
+            is_company_manager=True,
+        )
+        return user
 
 
 class ConversationSerializer(serializers.ModelSerializer):