WF_SECRET_FILE is ignored and WF_SECRET_KEY is still required

[magiccyril]

Describe the bug
When using the WF_SECRET_FILE environment variable, the application still requires WF_SECRET_KEY to be set. It seems that WF_SECRET_FILE is not taken into account at all.

To Reproduce
Steps to reproduce the behavior:

1. Deploy Wealthfolio with WF_SECRET_FILE set (pointing to a valid file containing the secret key)
2. Do not set WF_SECRET_KEY
3. Start the application

Expected behavior
If WF_SECRET_FILE is provided, the application should read the secret key from the file and not require WF_SECRET_KEY to be set as an environment variable.

Actual behavior
The application fails to start and reports that WF_SECRET_KEY is missing, even though WF_SECRET_FILE is defined and the file exists.

Environment
Deployment method: Docker Compose
Wealthfolio version : v2.1.0

Additional context
This makes it impossible to rely on Docker secrets (or similar mechanisms) using *_FILE environment variables, which is a common best practice for handling sensitive data.