Revert OIDC client secrets to file-based approach

Bcrypt hashes contain $ signs which Portainer interpolates when storing
as env vars, truncating the values. Use {{ secret "file" }} template
syntax instead — hashes live in /srv/authelia/config/secrets/ on the
host, written via Python to avoid shell interpolation.

Only $ -safe values (hex strings) remain as env vars.

authelia/config/configuration.yaml

@@ -65,7 +65,7 @@ identity_providers:
     clients:
       - client_id: open-webui
         client_name: Open WebUI
-        client_secret: '{{ env "AUTHELIA_OIDC_CLIENT_SECRET_OPEN_WEBUI" }}'
+        client_secret: '{{ secret "/config/secrets/oidc_open_webui" }}'
         public: false
         authorization_policy: one_factor
         token_endpoint_auth_method: client_secret_post
@@ -79,7 +79,7 @@ identity_providers:
 
       - client_id: linkding
         client_name: Linkding
-        client_secret: '{{ env "AUTHELIA_OIDC_CLIENT_SECRET_LINKDING" }}'
+        client_secret: '{{ secret "/config/secrets/oidc_linkding" }}'
         public: false
         authorization_policy: one_factor
         token_endpoint_auth_method: client_secret_post