Tighten role-based access control and expose a permissions endpoint

[MateoLostanlen]

Some sensitive endpoints (alert acknowledgement, camera live stream) need clearer role gating so that USER accounts cannot perform
agent/admin-only actions. The platform also needs a reliable way to know, ahead of time, whether the current user can perform a given
action.

Goals

• Enforce role checks on sensitive actions and ensure USER is blocked on:
  • Alert acknowledgement (sequence labeling, sequence unmatch)
  • Camera live stream access
• Return a clean, machine-readable error payload when access is denied, so the platform can display a proper message to the end user
  (e.g. consistent detail + an error code like forbidden_role).
• Add a GET /users/me/permissions (or equivalent) endpoint that returns, for the authenticated user, which actions they are allowed to
  perform (e.g. can_acknowledge_alert, can_access_live_stream, can_label_sequence, …). The frontend should rely on this rather than
  hardcoding role logic.