Add /available endpoint to trac-gateway

[fintrac-alex-hewson]

Summary

Adds a new `/availablez` endpoint to the gateway that reports the availability of each configured backend service. No authentication is required to call it.

Behaviour

• Probes each enabled backend service (metadata, data, orchestrator, admin) by attempting a TCP connection
• Returns plain text, one line per service:

  metadata=yes
  data=yes
  orchestrator=no
  

• Probes run in parallel; 2-second per-service connect timeout, 5-second total timeout
• Only services configured and enabled in `PlatformConfig` are included in the output

Implementation

• `AvailabilityHandler` (new) — `ChannelInboundHandlerAdapter` following the same internal-route pattern as `HealthCheckHandler`; receives `PlatformConfig` to resolve service addresses via `RoutingUtils`
• `RouteBuilder` — registers `/available` as an `INTERNAL` route (GET only), consistent with `/healthz`
• `InternalProxyBuilder` — wires `AvailabilityHandler` for the `available` protocol; receives `PlatformConfig` to pass through
• `Http1Router` — carries `PlatformConfig` to pass to `InternalProxyBuilder`
• `TracPlatformGateway` — stores `platformConfig` as an instance field and passes it when constructing `Http1Router`

Test plan

• CI build passes
• Call `GET /available` with no auth — returns `200 OK` with plain-text body listing services
• With all services running, all lines show `=yes`
• With a service stopped, its line shows `=no` and the response still arrives within the timeout