Lychee: Path traversal in `SecurePathController` allows authenticated arbitrary file read

Lychee is a free, open-source photo-management tool. Prior to 7.6.4, SecurePathController, which serves GET /image/{path}, concatenates the attacker-controlled path with the local storage root and performs a string-prefix check before canonicalizing dot segments. Under the supported non-default configuration with temporary_image_link_enabled enabled, temporary_image_link_when_logged_in disabled, and secure_image_link_enabled disabled, any authenticated user can submit encoded traversal segments that pass the prefix check and are later resolved by filesystem operations. This permits reading files available to the web process, including application configuration and other users' private images, and disclosed APP_KEY or database credentials can enable further compromise. This issue is fixed in version 7.6.4.