Add Authentik OIDC login

Same Authlib-based pattern as v1: SessionMiddleware + core/auth.py
(login/callback/logout + require_login dependency), gating "/",
/asr/* and /tts/* behind Authentik (application slug "asr-v2",
provider pk 14). The websocket route can't use a FastAPI dependency
(no Request in its scope) so it's split into its own router and
checks websocket.session manually before accept().

v2 has no public domain yet, so the redirect_uri points at the
internal 172.30.1.41:18101 address over plain http — hence
https_only=False on the session cookie. Client id/secret and the
session secret live in /srv/asr/env/asr.env under ASR_V2_-prefixed
names (that env file is shared with v1, which already owns the
unprefixed OIDC_* names).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

envs/gateway.txt

@@ -5,3 +5,5 @@ httpx==0.27.2
 websockets>=12.0
 jinja2==3.1.4
 numpy
+authlib==1.3.2
+itsdangerous==2.2.0