feat(gitea): add Anubis anti-crawler sidecar

Anubis (https://anubis.techaro.lol) is a lightweight proof-of-work
challenge that protects web services from AI crawlers and scrapers.

Changes:
- Add Anubis as an extraContainers sidecar in the Gitea HelmRelease
  - Listens on :8080, proxies to Gitea at http://localhost:3000
  - DIFFICULTY=4 (default; tune up to increase challenge strength)
  - SERVE_ROBOTS_TXT=true (Anubis manages robots.txt)
  - OG_PASSTHROUGH=true (OpenGraph bots pass through for link previews)
  - Signs challenges with ED25519 key from 'anubis-key' secret
- Add anubis port (8080) to Gitea service via additionalPorts
- Update Ingress to route to the 'anubis' port instead of 'http'
- Add anubis-secret.yaml placeholder with creation instructions

One-time setup required before applying:
  kubectl create secret generic anubis-key \
    --namespace gitea \
    --from-literal=ED25519_PRIVATE_KEY_HEX=$(openssl rand -hex 32)

manifests/gitea/kustomization.yaml

@@ -7,3 +7,4 @@ resources:
   - ingress.yaml
   - secrets.yaml
   - dump-cronjob.yaml
+  - anubis-secret.yaml