feat: CORS so browsers can fetch shell config cross-origin

Apps run on different hostnames (crm.gosec.internal,
chronos.gosec.internal, …) so the browser fetch from <AppShell> to
shell-api.gosec.internal is cross-origin and was being blocked.

Add Fiber's cors middleware. Allowlist: any *.gosec.internal or
*.gosec.cloud origin (override via CORS_ORIGINS env if a tighter
list is needed). Allow Authorization + Content-Type + If-None-Match
on the request side; expose ETag on the response side. Methods
limited to GET + OPTIONS.

Image: v0.1.4

Verified:
  curl -X OPTIONS https://shell-api.gosec.internal/api/v1/shell/gsc-crm
    -H "Origin: https://crm.gosec.internal" → 204 with
    access-control-allow-origin: https://crm.gosec.internal

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

k8s/deployment.yaml

@@ -18,7 +18,7 @@ spec:
     spec:
       containers:
         - name: api
-          image: registry.gosec.internal/gsc-shell-api:v0.1.3
+          image: registry.gosec.internal/gsc-shell-api:v0.1.4
           imagePullPolicy: IfNotPresent
           ports:
             - name: http