feat(gateway-api): GitHub SSO via oauth2-proxy in front of dsh-gateway (#13)

Add oauth2-proxy (GitHub provider, org-restricted) as a reverse proxy in
front of the dsh-gateway, wired through the Traefik Gateway API. The gateway
inherits the authenticated GitHub username via X-Forwarded-User (SSO).

- gateway.py: opt-in AUTH_PROXY_HEADER trust, remote_user(), SSO bootstrap
  (auto-provision claim + mint/rotate token + session cookie), patch_token()
- 50-gateway.yaml: AUTH_PROXY_HEADER=X-Forwarded-User, COOKIE_SECURE=true
- gateway-api/dsh-demo: oauth2-proxy Deployment/Service + secret example,
  HTTPRoute backend -> oauth2-proxy:4180
- tests: 10 new SSO cases (106 passing)

Placeholders (dsh-demo.example.com, your-github-org) are substituted at
apply time.

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

gateway-api/dsh-demo/oauth2-proxy.yaml

@@ -0,0 +1,119 @@
+# oauth2-proxy — GitHub OAuth login in front of the dsh-gateway.
+#
+# Runs as a reverse proxy: it authenticates the browser against GitHub
+# (restricted to a configured organization), then forwards the request to
+# the dsh-gateway Service with the authenticated identity in X-Forwarded-User
+# (plus X-Auth-Request-* via --set-xauthrequest). The gateway trusts that
+# header (AUTH_PROXY_HEADER) to inherit the username (SSO).
+#
+# Credentials come from the "oauth2-proxy" Secret (see
+# oauth2-proxy-secret.yaml.example); this manifest never carries secrets.
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: oauth2-proxy
+  namespace: dsh-demo
+  labels:
+    app: oauth2-proxy
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: oauth2-proxy
+  template:
+    metadata:
+      labels:
+        app: oauth2-proxy
+        app.kubernetes.io/part-of: dsh-demo
+    spec:
+      securityContext:
+        runAsNonRoot: true
+        runAsUser: 2000
+        runAsGroup: 2000
+        seccompProfile:
+          type: RuntimeDefault
+      containers:
+        - name: oauth2-proxy
+          image: quay.io/oauth2-proxy/oauth2-proxy:v7.15.5
+          imagePullPolicy: IfNotPresent
+          securityContext:
+            allowPrivilegeEscalation: false
+            readOnlyRootFilesystem: true
+            capabilities:
+              drop: ["ALL"]
+          args:
+            - --http-address=0.0.0.0:4180
+            - --provider=github
+            # TODO: replace with the GitHub organization allowed to sign in.
+            - --github-org=your-github-org
+            - --email-domain=*
+            # Reverse-proxy target: the gateway backend Service.
+            - --upstream=http://dsh-gateway:8080
+            # Public callback (TLS terminates at the Gateway listener).
+            # TODO: replace with the actual public FQDN.
+            - --redirect-url=https://dsh-demo.example.com/oauth2/callback
+            - --cookie-secure=true
+            - --cookie-name=_oauth2_proxy
+            # Also expose X-Auth-Request-User/Email/Groups upstream.
+            - --set-xauthrequest
+            # Single provider: skip the provider-selection page.
+            - --skip-provider-button
+          env:
+            - name: OAUTH2_PROXY_CLIENT_ID
+              valueFrom:
+                secretKeyRef:
+                  name: oauth2-proxy
+                  key: client-id
+            - name: OAUTH2_PROXY_CLIENT_SECRET
+              valueFrom:
+                secretKeyRef:
+                  name: oauth2-proxy
+                  key: client-secret
+            - name: OAUTH2_PROXY_COOKIE_SECRET
+              valueFrom:
+                secretKeyRef:
+                  name: oauth2-proxy
+                  key: cookie-secret
+          ports:
+            - name: http
+              containerPort: 4180
+          livenessProbe:
+            httpGet:
+              path: /ping
+              port: http
+            initialDelaySeconds: 5
+            periodSeconds: 10
+            timeoutSeconds: 1
+            failureThreshold: 3
+          readinessProbe:
+            httpGet:
+              path: /ready
+              port: http
+            initialDelaySeconds: 5
+            periodSeconds: 10
+            timeoutSeconds: 5
+            failureThreshold: 3
+          resources:
+            requests:
+              cpu: 10m
+              memory: 32Mi
+            limits:
+              cpu: 200m
+              memory: 128Mi
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: oauth2-proxy
+  namespace: dsh-demo
+  labels:
+    app: oauth2-proxy
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  selector:
+    app: oauth2-proxy
+  ports:
+    - name: http
+      port: 4180
+      targetPort: http