[Task](heureka): Implement change severity action, modal, and vulnerability state update between tabs

[hodanoori]

Acceptance Criteria

• A change severity action is available for eligible vulnerabilities.
• Selecting the action opens a change severity modal with required inputs.
• Submitting the modal successfully records the changed severity.
• The vulnerability state is updated accordingly and reflected between the Active and Vulnerabilities tabs.
• UI updates immediately after submission without page reload.