feat(dex): add forgejo-runner-sizer OIDC static client

Register forgejo-runner-sizer as a Dex static client for OIDC
authentication on sizer.dev.t09.de. Adds the client secret env var
injection and the staticClients entry with secretEnv reference.

otc/dev.t09.de/stacks/core/dex/values.yaml

@@ -34,6 +34,11 @@ envVars:
       secretKeyRef:
         name: dex-argo-client
         key: clientSecret
+  - name: FORGEJO_RUNNER_SIZER_CLIENT_SECRET
+    valueFrom:
+      secretKeyRef:
+        name: dex-runner-sizer-client
+        key: clientSecret
   - name: LOG_LEVEL
     value: debug
 
@@ -74,3 +79,8 @@ config:
         - "https://grafana.dev.t09.de/login/generic_oauth"
       name: "Grafana"
       secretEnv: "OIDC_DEX_GRAFANA_CLIENT_SECRET"
+    - id: forgejo-runner-sizer
+      name: "Forgejo Runner Sizer"
+      redirectURIs:
+        - "https://sizer.dev.t09.de/ui/callback"
+      secretEnv: "FORGEJO_RUNNER_SIZER_CLIENT_SECRET"