feat: use secret_key config field instead of api_key (#2444)

Fixes #2426

docker/config.dev.yml

@@ -1,6 +1,7 @@
 general:
   public_url: http://localhost:8080
   api_key: some_secret_api_key
+  secret_key: some_secret_key
 
 database:
   host: postgres

docker/config.yml

@@ -7,6 +7,10 @@ general:
   # The internal API authentication key.
   # > this field is REQUIRED
   api_key:
+  # The Django API secret key. If not defined, the value of [general.api_key] will be
+  # used as fallback.
+  # > this field will be REQUIRED starting with LibreTime 4.0.0
+  secret_key:
 
   # List of origins allowed to access resources on the server, the public url
   # origin is automatically included.

docker/example/config.yml

@@ -7,6 +7,10 @@ general:
   # The internal API authentication key.
   # > this field is REQUIRED
   api_key: some_secret_api_key
+  # The Django API secret key. If not defined, the value of [general.api_key] will be
+  # used as fallback.
+  # > this field will be REQUIRED starting with LibreTime 4.0.0
+  secret_key:
 
   # List of origins allowed to access resources on the server, the public url
   # origin is automatically included.