fix(security): prevent path traversal in user data deletion

[tkiran]

Problem: UserManager.remove_user() built user_path = self.data_path / user_id and then called shutil.rmtree(user_path). Because user_id is user-controlled, an attacker could supply traversal/absolute paths (e.g., ../.., C:\Windows, \server\share) to delete directories outside the intended user data directory.

Impact: Arbitrary recursive directory deletion (critical), potentially wiping system files or other application data.

Fix: Added strict user_id validation and a safe path resolver that:

• rejects traversal, absolute/UNC/drive paths, separators, and null bytes
• resolves paths and enforces that the target directory is contained within self.data_path (prevents traversal and symlink escapes)
• uses this safe path in remove_user (and consistently across related user file operations)

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