infra: Expose Ollama service via Traefik

Adds the `traefik.enable=true` tag to the `ollama` service definition within the `ai-backend` Nomad job.

This enables Traefik to automatically discover and route incoming requests to the Ollama service, making it accessible through the Traefik ingress.

stacks/ai/ai-backend.nomad

@@ -49,6 +49,7 @@ job "ai-backend" {
       service {
         name = "ollama"
         port = "api"
+        tags = ["traefik.enable=true"]
         check {
           type     = "http"
           path     = "/"