Add target to pool

Adds a backend target (member) to one of the load balancer's pools via the "Add target" dialog: target name, destination pool (poolId), IP address and port (defaulting to 8080). The member is created with weight 1 and backupMember=false unless overridden, begins receiving health checks from the pool's monitor, and appears in the Targets tab and the pool's members array. Validation: parent load balancer must exist in the scope (404 NOT_FOUND); poolId must reference an existing pool of this LB — the dialog is disabled when no pools exist, and the backend rejects an unknown pool with 404; name and address non-empty, address a valid IP reachable from the VIP subnet and port an integer 1-65535 (422 VALIDATION_FAILED); the same address:port pair must not already be a member of the target pool (409 CONFLICT); weight a positive integer when supplied (422). Async & state: Asynchronous — returns 202 with the LbMember resource; initial operating state reflects the first health probe (ONLINE once the monitor passes, NO_MONITOR when the pool has no monitor) while the parent LB passes through provisioning=PENDING_UPDATE → ACTIVE. resource.updated SSE frames (kind "network.load_balancer", patch to pools[].members and the healthy-target counters) refresh the UI. Errors: 401 unauthenticated; 403 caller lacks contributor/owner/admin; 404 unknown LB or pool; 409 duplicate address:port in the pool or LB state conflict; 422 invalid name/address/port/weight; 500 backend failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the originally created member instead of registering the target twice.