Harden secret key configuration

app/__init__.py

@@ -3,7 +3,7 @@ from pathlib import Path
 from flask import Flask, flash, render_template
 from werkzeug.exceptions import RequestEntityTooLarge
 
-from app.config import Config
+from app.config import Config, validate_secret_key
 from app.routes import main_blueprint
 from app.services.storage import cleanup_expired_outputs
 
@@ -21,6 +21,7 @@ def create_app(config_class: type[Config] = Config) -> Flask:
     """Application factory used by Flask and Gunicorn."""
     app = Flask(__name__, instance_relative_config=True)
     app.config.from_object(config_class)
+    validate_secret_key(app.config["SECRET_KEY"])
 
     output_dir = Path(app.config["OUTPUT_DIRECTORY"])
     if not output_dir.is_absolute():