Supports login cross multiple RAGFlow servers (#13322)

### What problem does this PR solve?

1. Use redis to store the secret key.
2. During startup API server will read the secret from redis. If no such
secret key, generate one and store it into redis, atomically.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/redis_conn.py

@@ -334,6 +334,42 @@ class RedisDB:
             self.__open__()
         return -1
 
+    def get_or_create_secret_key(self, key_name: str, new_value: str) -> str:
+        """
+        Atomically get an existing key or create a new one.
+
+        This method guarantees that across multiple concurrent calls, only one
+        key will be created and all callers will receive the same key.
+
+        Returns:
+            The secret key string
+
+        Raises:
+            redis.RedisError: If Redis operations fail
+        """
+        # First, try to get the existing key
+        existing_value = self.REDIS.get(key_name)
+        if existing_value is not None:
+            logging.debug("Retrieved existing key from Redis")
+            return existing_value
+
+        # Use SETNX to atomically set the key only if it doesn't exist
+        # SETNX returns True if the key was set, False if it already existed
+        if self.REDIS.setnx(key_name, new_value):
+            logging.info("Successfully created new secret key in Redis")
+            return new_value
+
+        # SETNX failed, meaning another process created the key concurrently
+        # Retrieve and return that key
+        final_key = self.REDIS.get(key_name)
+        if final_key is None:
+            # This should rarely happen, but retry if it does
+            logging.warning("Key disappeared during concurrent access, retrying...")
+            return self.get_or_create_secret_key(key_name, new_value)
+
+        logging.debug("Retrieved key created by another process")
+        return final_key
+
     def transaction(self, key, value, exp=3600):
         try:
             pipeline = self.REDIS.pipeline(transaction=True)