feat(socket-auth): add customer.socketToken() for realtime channels

[roncodes]

What

• Customer#socketToken(params?, options?) calls POST customers/socket-token with the customer's Customer-Token header (request-scoped, via performAuthorizedRequest) and resolves to { token, expires_in, expires_at }.
• New exported type SocketTokenResponse.
• Declarations in types/ updated to match; public API snapshot gains Customer.socketToken.
• README: Realtime order updates section (in-memory authEngine, socket.authenticate() refresh ~60 s before expiry, subscribing to order.{id}, the checkout socket_token); docs/api-contract.md lists the route.
• No version bump.

Why

Realtime channels are moving to authenticated subscriptions. Storefront customers need a token scoped to their own channels instead of subscribing anonymously.

Test plan

• tests/actions.test.js covers the endpoint, method, response and the Customer-Token header (existing loop asserts it on every authorized call); public API snapshot updated.
• dist/ is generated output. It was not rebuilt locally; a follow-up commit on this branch regenerates it, otherwise the Require reproducible generated artifacts step will flag it.
• Verified by CI.

Related PRs

Part of the authenticated realtime channels rollout (socket auth), one PR per repo:

• feat(socket-auth): authenticated realtime channels (tokens, authorizer, signed publish) core-api#290 — tokens, channel authorizer, signed HTTP publish (merge and tag first: fleetops and storefront require fleetbase/core-api ^1.6.69)
• feat(socket-auth): fleetops channel resolvers, driver socket tokens, per-customer tracking channel fleetops#358 — fleetops channel resolvers, driver socket tokens, per-customer tracking channel
• feat(socket-auth): storefront customer and checkout socket tokens, channel resolvers storefront#113 — customer and checkout socket tokens, storefront/checkout resolvers
• feat(socket-auth): Fleetbase socket server with JWT auth and signed internal publish fleetbase#704 — fleetbase/fleetbase-socket server, compose/helm/installer, console socket test page
• feat(socket-auth): authenticate the realtime socket with session socket tokens ember-core#97 — console socket service authenticates with session tokens
• feat(socket-auth): show channel authorization failures in the sockets viewer dev-engine#51 — sockets viewer shows authorization failures
• feat(socket-auth): authenticate realtime socket with driver socket tokens navigator-app#112 — driver app socket tokens
• feat(socket-auth): authenticate realtime socket with customer and checkout socket tokens storefront-app#65 — customer and checkout socket tokens
• feat(socket-auth): generate the socket auth key on install fleetbase-cli#11 — installer generates the socket auth key
• feat(socket-auth): add socket.token() to mint realtime socket tokens fleetbase-js#45, feat(socket-auth): add customer.socketToken() for realtime channels #22, feat(socket-auth): add socket->token() to mint realtime socket tokens fleetbase-php#28 — SDK socket token helpers
• docs(socket-auth): authenticated realtime channels and migration guide fleetbase.io#120 — docs and migration guide