Activate organization.

Exchanges an unassigned activation key for activation of the authenticated user's current inactive organization. The key proves system-issued activation provenance; it is not pre-bound to an organization.

Requires cookie authentication, but intentionally does not require an active organization because the target organization is not active yet. The route is rate limited and returns a generic validation failure when the key is invalid, expired, already used, revoked, or the current organization is not eligible.