[Feature]: Add checksum and integrity support for uploaded files

[DenizAltunkapan]

Problem

Cloud Page currently stores files on disk, but there is no explicit checksum or integrity metadata for uploaded files.

This makes it harder to detect corrupted files, support future sync logic, identify duplicate uploads, or verify that a download matches the stored file.

Proposed solution

Calculate and expose a checksum for files, starting with SHA-256.

Scope:

• calculate a SHA-256 checksum when a file is uploaded or overwritten
• expose checksum metadata through file metadata/listing responses where appropriate
• add a way to recalculate or verify a checksum for an existing file
• keep checksum handling scoped to the authenticated user's root folder

Example scenario: a user uploads a large document and later downloads it again. Cloud Page can return the stored checksum so the client or future sync logic can verify integrity.

Acceptance criteria

• Uploaded files get a SHA-256 checksum calculated server-side.
• Checksum data is available through an API response without exposing host filesystem paths.
• Existing files can be verified or rehashed safely.
• Path traversal and cross-user access are prevented during checksum operations.
• Tests cover upload checksum creation, checksum retrieval, and denied access outside the user root.