build: run the admin UI from a container, like everything else

[PhilippTheServer]

Closes #13

The admin UI was the only part of OpenTaberna you could not docker compose up. Now it
matches OpenTaberna/frontend exactly: Dockerfile, nginx.conf, compose.yaml,
.dockerignore, multi-stage build, nginx on 4200.

The API is now /api, not a hardcoded host

apiBaseUrl was http://localhost:8000, baked into the bundle. It is now the same-origin
path /api, mapped two ways:

• container — nginx proxies /api/ → http://host.docker.internal:8000/, same as the storefront
• ng serve — proxy.conf.json does the same, so npm start keeps working

Both behave identically and neither ships an API host. The Keycloak URL stays absolute,
because the browser is redirected there and a same-origin path would be meaningless.

Verification

$ npx ng test --watch=false
 Test Files  10 passed (10)
      Tests  67 passed (67)

$ docker compose up -d
$ curl -s http://localhost:4200/ | grep -o "<title>.*</title>"
<title>OpenTaberna Admin</title>

$ curl -o /dev/null -w "%{http_code}" http://localhost:4200/api/v1/items/
200
$ curl -o /dev/null -w "%{http_code}" http://localhost:4200/api/v1/admin/analytics/summary
403          ← auth still enforced through the proxy

And ng serve still reaches the API through the new dev proxy:

$ npx ng serve --port 4201
$ curl http://localhost:4201/api/v1/items/
{"success":true,"message":"Items retrieved successfully",…}

One thing I found and did not fix here

src/environments/environment.prod.ts is never referenced. angular.json has no
fileReplacements, so ng build --configuration production silently uses environment.ts
— which is why the hardcoded http://localhost:8000 mattered at all.

This change removes the immediate harm (there is no host to get wrong now), but the dead
file is still misleading: it carries a real-looking https://auth.opentaberna.de Keycloak
URL that no build has ever used. Worth either wiring up or deleting, in its own change.