anubis: bot-challenge proxy in front of the guarded vhosts (two-tier chain)

Bots and AI scrapers hit the SSO-fronted vhosts and eat Authentik auth
work for 403s. Anubis verifies clients first: DENY for known AI scraper
UAs, JS challenge for everyone else; verified traffic resumes the normal
Authentik forward_auth chain on Caddy's internal listener (:8016).

Chain: client -> Caddy (TLS) -> Anubis :8923 (verify) -> Caddy :8016
(auth + routing resumes; Host survives both hops) -> app. Reverse-proxy
through Anubis, not forward_auth: at the pinned tag a CHALLENGE answers
HTTP 200 + HTML, and forward_auth treats 200 as "authenticated" — the
page would pass through unchallenged. Exempt by construction (never
routed through Anubis): plex (plain HTTP legacy), auth.* (IdP login
must be reachable ungated), gitea (git smart-HTTP clients cannot solve
JS challenges), and every app with its own login.

Pinned ghcr.io/TecharoHQ/anubis:v1.13.0 — the tag was source-verified
before pinning: at this tag the policy parser is JSON-only with a
minimal bots[] schema (no imports, no CEL expressions), flags are the
config surface via flagenv (TARGET/POLICY_FNAME map to --target/
--policy-fname), and the image's exec-form HEALTHCHECK is inherited by
the quadlet unit (dodges the v1 generator's HealthCmd space-split quirk).

Rollback nuance: stopping anubis 502s everything behind the chain (the
public listeners proxy through it); a true rollback is reverting the
Caddyfile and redeploying, not just systemctl --user stop anubis.

dankclaw/anubis/policy.json

@@ -0,0 +1,15 @@
+{
+  "bots": [
+    {
+      "name": "deny AI scrapers",
+      "user_agent_regex": "(?i)^(GPTBot|ChatGPT-User|OAI-SearchBot|CCBot|anthropic-ai|ClaudeBot|Claude-Web|PerplexityBot|PerplexityUser|Google-Extended|Applebot-Extended|Bytespider|Amazonbot|meta-externalagent|DuckAssistBot|cohere-ai|YouBot|KimiBot)",
+      "action": "DENY"
+    },
+    {
+      "name": "challenge everything else",
+      "path_regex": ".",
+      "action": "CHALLENGE"
+    }
+  ],
+  "dnsbl": false
+}