feat(traefik-forward-auth): serve several domains and portals from one instance

A session cookie scoped to one registrable domain is never sent to a host under
another, so an app on a second domain could not be protected by this instance at
all. It failed with a 500 naming the cause: "return URL host does not match any
configured cookie domain".

`domains` is now a list of {domain, authHost}, matching the upstream
`server.domains` schema. The chart previously emitted `server.hostname` and
`cookies.domain`, which are not in 4.14.1's documented options — a deprecated
form that still worked. The single-domain values remain as shorthand and fold
into one entry, so an existing release renders the same protection it did before.

`portals` is a list too. A portal is one OAuth2 client, chosen by the middleware
address, so two portals means two Pocket ID applications — which is how access
can be separated per domain rather than merely shared across them. Each portal
gets its own Middleware and its own mounted secret, under
/var/run/secrets/traefik-forward-auth/<portal>/.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

traefik-forward-auth/Chart.yaml

@@ -2,7 +2,7 @@ apiVersion: v2
 name: traefik-forward-auth
 description: Forward auth for Traefik (ItalyPaleAle/traefik-forward-auth) with Pocket ID provider
 type: application
-version: 0.1.6
+version: 0.2.0
 appVersion: "4.14.1"
 annotations:
   version-source: github-release:ItalyPaleAle/traefik-forward-auth