Add Change Password Endpoint

[ayshadogo]

Description:
Allow authenticated users to change their own password.

Tasks:

• Create PATCH /api/auth/change-password
• Require authentication
• Validate currentPassword and newPassword
• Verify current password; return 401 if incorrect
• Hash and update new password; invalidate all refresh tokens

Acceptance Criteria:
Authenticated user can change password; wrong current password returns 401.

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