GHSA-7rx3-5wx3-5v76: Nebula-mesh allows non-admin operators to disable webhook SSRF protection via `allow_private`

Non-admin operators (role user) can set allow_private: true on their own managed webhook subscription (POST/PATCH /api/v1/webhook-subscriptions). No admin check exists on this field. At delivery time, allow_private switches the dispatcher to an unguarded HTTP client, bypassing the private/loopback/link-local SSRF guard — letting a low-privilege operator make the server request internal addresses.