fix(api): Allow unlinking external issues with event:write

[betegon]

Allow event:write to unlink existing external issues through native integrations and both Sentry App DELETE routes. Users who can link a resource can also unlink it, including project members whose organization disables eventsMemberAdmin.

The group routes use a narrow GroupPermission subclass; the installation route updates its existing permission map. event:admin remains accepted, and deleting an actual Sentry issue still requires it. Existing organization, project and App identity checks remain in place. Paths, link deletion behavior and RPC contracts are unchanged.

There are existing precedents for DELETE with write permission:

• ProjectTeamsPermission allows removing a project/team association with project:write.
• ExternalUserPermission allows removing an external-user association with org:write.
• ReplayDetailsPermission allows deleting a replay with project:write.

The issue PUT and native integration PUT already accept event:write. This extends that policy specifically to removing external references; the shared issue DELETE permission is unchanged.

[betegon]

The optional openapi-diff check exits 1 on the intended event:write scope additions. Schema generation and API docs tests pass.