docs: add required Redis mode parameter to v2 authentication docs

docs/v2/configuration/authentication.mdx

@@ -116,6 +116,30 @@ authentication:
     storage:
       type: redis
       redis:
+        mode: single # Required: either "single" or "cluster"
+        host: localhost
+        port: 6379
+        db: 0
+        password: password
+```
+
+The `mode` parameter is **required** and must be set to either:
+
+- `single` - For standalone Redis instances (most common)
+- `cluster` - For Redis cluster setups
+
+##### Redis Cluster Example
+
+For Redis cluster configurations, use `mode: cluster`:
+
+```yaml config.yaml
+authentication:
+  required: true
+  session:
+    storage:
+      type: redis
+      redis:
+        mode: cluster
         host: localhost
         port: 6379
         db: 0

docs/v2/configuration/overview.mdx

@@ -315,6 +315,7 @@ Authentication configuration controls how users and systems authenticate with Fl
 
 | Property                                                | Description                                                         | Default   | Since  |
 | ------------------------------------------------------- | ------------------------------------------------------------------- | --------- | ------ |
+| authentication.session.storage.redis.mode               | **Required** Redis mode: `single` or `cluster`                      | single    | v2.0.0 |
 | authentication.session.storage.redis.host               | Host to access the Redis database                                   | localhost | v2.0.0 |
 | authentication.session.storage.redis.port               | Port to access the Redis database                                   | 6379      | v2.0.0 |
 | authentication.session.storage.redis.db                 | Redis database to use                                               | 0         | v2.0.0 |