Add static route

Adds a static route to the router's route table (the "Add route" action and inline draft row on the Static routes tab of the router detail page, and the optional initial routes section of the router creation form). The caller supplies a destination CIDR and a nexthop IP; on acceptance the route appears in the Static routes tab and in the effective Route table in lookup order after connected-subnet routes. Validation: router must exist and be visible in scope → 404 NOT_FOUND; destination is required and must be a syntactically valid CIDR (e.g. 10.20.0.0/16) → 422 VALIDATION_FAILED; nexthop is required and must be a valid IP address reachable via one of the router's attached interfaces → 422 on malformed or unreachable next hop, and of the same IP version as destination → 422 on nexthop otherwise; a route with the same destination CIDR must not already exist on this router → 409 CONFLICT. Async & state: Asynchronous — returns 202 with the accepted StaticRoute (id assigned by the backend); dataplane route-table programming completes shortly after, signalled by a resource.updated SSE frame with kind: network.router patching the parent router's routes array. Terminal outcome: route present and effective, or removed with the failure reported on the router activity feed. Errors: 401 unauthenticated; 403 caller lacks contributor/owner/admin rights; 404 unknown router; 409 duplicate destination CIDR on this router; 422 missing/malformed destination or next hop; 500 internal failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the original 202 response with the same route id instead of inserting a duplicate row — retries are made safe where duplicate creation would be harmful.