Pin verified versions; add decision tree for Authelia coverage

- Pin AUTHELIA_VERSION=4.39.19 (current stable, released 2026-04-12) and
  FAIL2BAN_VERSION=1.1.0-r0 in .env.example + docker-compose.yml.
- Reshape access_control.rules and the Caddyfile snippet around a
  three-case decision tree: no app auth (Authelia is the gate), app with
  proxy-auth support (switch FROM app login TO Authelia headers), and
  apps that keep their own login (skip Authelia entirely).
- Document Frigate 0.14+ proxy auth specifically: auth.enabled: False,
  proxy.header_map (remote-user / remote-groups), trusted_proxies for
  the caddy_net subnet, optional X-Proxy-Secret for cross-VLAN trust.

https://claude.ai/code/session_013XZ1vmgk78k2PEQ5DmJhF3

.env.example

@@ -7,8 +7,8 @@
 # Pin your image versions. Bump to current stable when you upgrade --
 # check https://github.com/authelia/authelia/releases and
 # https://github.com/crazy-max/docker-fail2ban/releases.
-AUTHELIA_VERSION=4.39
-FAIL2BAN_VERSION=latest
+AUTHELIA_VERSION=4.39.19
+FAIL2BAN_VERSION=1.1.0-r0
 
 # Used by both containers for log timestamps. Set to your IANA zone.
 TZ=America/New_York