docs: document outpost router + priority gotcha for forward-auth

The Traefik example was missing the veola-outpost router (PathPrefix
/outpost.goauthentik.io/ -> authentik service) and the header-strip
middleware, so a fresh deploy following it would 404 on Sign out.

Add both, plus a prominent note: the outpost router must outrank the
catch-all Host(...) router. Traefik's default priority is rule length,
so Authentik's docs value of priority:15 silently loses once the
hostname rule exceeds 15 chars (e.g. veola.parodia.dev = 25), letting
the catch-all swallow /outpost.goauthentik.io/sign_out into Veola's
404. Use a high explicit priority (100).

deploy/authentik-forward-auth.md

@@ -47,19 +47,58 @@ http:
           - X-authentik-email
           - X-authentik-name
           - X-authentik-groups
+    # Strip client-supplied identity headers on ingress so only the outpost
+    # can set them (runs before `authentik` on the protected router).
+    authentik-strip:
+      headers:
+        customRequestHeaders:
+          X-authentik-username: ""
+          X-authentik-email: ""
+          X-authentik-name: ""
+          X-authentik-groups: ""
+          X-authentik-uid: ""
 
   routers:
     veola:
       rule: "Host(`veola.example.com`)"
       service: veola
       middlewares:
+        - authentik-strip
         - authentik
       tls:
         certResolver: letsencrypt
+
+    # The outpost's own endpoints (callback, start, sign_out) must go straight
+    # to Authentik, NOT through the forwardAuth-protected `veola` router. This
+    # router has NO auth middleware and points at the authentik service.
+    veola-outpost:
+      rule: "Host(`veola.example.com`) && PathPrefix(`/outpost.goauthentik.io/`)"
+      priority: 100
+      service: authentik
+      tls:
+        certResolver: letsencrypt
+
+  services:
+    authentik:
+      loadBalancer:
+        servers:
+          - url: "http://authentik-server:9000/"
 ```
 
-Traefik must strip any client-supplied `X-Authentik-*` and `X-Forwarded-*`
-headers on ingress so only the outpost can set them.
+**Router priority — important.** Both routers match the Veola host, so the more
+specific `veola-outpost` must win. Traefik's *default* priority is the rule
+length, so the catch-all `veola` router (`Host(...)`) gets a priority equal to
+its length (e.g. `Host(`veola.parodia.dev`)` = 25). Authentik's docs example
+sets the outpost router to `priority: 15`, which **silently loses** whenever the
+hostname rule is longer than 15 chars — the catch-all then swallows
+`/outpost.goauthentik.io/*` and hands it to Veola, which 404s. Symptom: clicking
+**Sign out** lands on `/outpost.goauthentik.io/sign_out` with a plain
+`404 page not found`. Fix: give `veola-outpost` a high explicit priority (e.g.
+`100`) so it always beats the catch-all.
+
+(Login still works even when the outpost router loses, because the outpost
+intercepts `callback`/`start` during the forwardAuth `/auth/traefik` round-trip;
+`sign_out` is the one path that genuinely needs the dedicated router.)
 
 ## Break-glass