Rewrite slash-less Authentik token endpoint to the canonical path (#478)

Authentik advertises the token endpoint with a trailing slash, but some OIDC clients (the ArgoCD iOS app) POST to /application/o/token without one; Django's APPEND_SLASH will not redirect a POST, so the token exchange gets 405 and login fails.

- Add an exact-match rule on /application/o/token to the authentik and authentik-internal HTTPRoutes.
- Rewrite it to /application/o/token/ with a URLRewrite ReplaceFullPath filter, preserving the method and the authentik-server backend.
- Leave the catch-all PathPrefix rule untouched; exact matches outrank it in Gateway API precedence.

Reviewed-on: #478
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/authentik/httproute.yaml

@@ -37,6 +37,22 @@ spec:
       name: authentik
       sectionName: https
   rules:
+    - backendRefs:
+        - group: ""
+          kind: Service
+          name: authentik-server
+          port: 80
+          weight: 1
+      filters:
+        - type: URLRewrite
+          urlRewrite:
+            path:
+              type: ReplaceFullPath
+              replaceFullPath: /application/o/token/
+      matches:
+        - path:
+            type: Exact
+            value: /application/o/token
     - backendRefs:
         - group: ""
           kind: Service
@@ -86,6 +102,22 @@ spec:
       name: authentik-internal
       sectionName: https
   rules:
+    - backendRefs:
+        - group: ""
+          kind: Service
+          name: authentik-server
+          port: 80
+          weight: 1
+      filters:
+        - type: URLRewrite
+          urlRewrite:
+            path:
+              type: ReplaceFullPath
+              replaceFullPath: /application/o/token/
+      matches:
+        - path:
+            type: Exact
+            value: /application/o/token
     - backendRefs:
         - group: ""
           kind: Service