authelia: add reusable per-service access scoping (universal vs specific users)

Every domain with an access_control rule was reachable by any Authelia
user by default (the existing catch-all *.${AUTHELIA_DOMAIN} rule) — no
way to restrict a specific service to a subset of users without hand-
editing configuration.yml and users.yml directly.

_authelia_scope_access(SERVICE_ID, DOMAIN) is a new generic, reusable
helper: call it after any service finishes being protected by Authelia
(forward_auth gate or native OIDC alike — it only cares about the domain).
Offers universal vs. specific-users access; if scoped, creates a
"<service_id>-only" group, adds every listed username to it (creating
accounts on the fly for names that don't exist yet, via the new
_authelia_create_user_noninteractive — a non-interactive sibling to
add_authelia_user, same extraction pattern already used for
_authelia_provision_oidc_client), and inserts an allow+deny rule pair
above the general catch-all. Idempotent on rerun.

_authelia_report_access_scope() (new menu option 6) is the read side —
lists who has universal vs. service-scoped access, and offers to promote
a scoped user back to universal by removing their "-only" group
membership.

services/gitea.sh's _gitea_offer_authelia_sso() is the reference
integration, calling _authelia_scope_access after successfully wiring up
Gitea's OIDC login. The other services with a plain "Protect X with
Authelia?" prompt (magicmirror, wolf-pair, js99er, drum-rhythm-game,
iopaint, paintplus, stirling-pdf, wolf) are natural follow-ups once this
is confirmed working live — each just needs one added call.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YEQNc4NfBST1m9NtCZVYa8

services/gitea.sh

@@ -236,6 +236,8 @@ _gitea_offer_authelia_sso() {
         log_warning "  Discovery URL:  $_discovery_url"
         log_warning "  (The Client Secret above is shown once — it isn't stored in plaintext anywhere.)"
     fi
+
+    declare -F _authelia_scope_access >/dev/null 2>&1 && _authelia_scope_access "gitea" "$GITEA_OIDC_DOMAIN"
 }
 
 # Offers to enable Gitea Actions (Gitea's own CI, largely GitHub-Actions-