fix(billing): make Cloud seat enforcement automatic (#7542)

Replace the separate Fleet/capacity URLs and seat-capacity switches with the single managed MULTICA_CLOUD_URL connection. Strict seat admission turns on whenever Multica is Cloud-connected and fails closed when the capacity machine token is absent or invalid; the recovery worker starts only for a valid executor so live intents keep their retry budget during credential outages.

Split invitation throttling into a non-consuming precheck and post-reservation consumption. Persistent capacity_full and capacity_overcommitted rejections charge only the actor budget; transient failures charge none. Preserve every Cloud/proxy HTTP 429 as retryable with both Retry-After forms, defer outbox rows without spending attempts, and use Cloud's rate-limit scope so a workspace 429 delays only that tenant while global or unscoped 429s stop the current batch.

BREAKING: delete/unset MULTICA_FLEET_URL, MULTICA_CLOUD_FLEET_URL, MULTICA_CLOUD_FLEET_TIMEOUT, MULTICA_SUBSCRIPTION_CAPACITY_ENABLED, MULTICA_SUBSCRIPTION_CAPACITY_URL and MULTICA_SUBSCRIPTION_CAPACITY_WORKER_ENABLED before rollout. The startup guard rejects any non-empty value, so MULTICA_SUBSCRIPTION_CAPACITY_ENABLED=false still blocks boot. Configure MULTICA_CLOUD_URL, MULTICA_CLOUD_TIMEOUT and MULTICA_SUBSCRIPTION_CAPACITY_SERVICE_TOKEN instead; entitlements keep their independent MULTICA_ENTITLEMENT_POLICY_* configuration.

Companion: multica-ai/multica-cloud#57