Wire Authelia SSO into Immich, Audiobookshelf, and Beszel

_authelia_provision_oidc_client gains an optional PKCE flag (new 5th
positional arg; every existing caller updated to pass "n", producing an
identical client block to before) — Audiobookshelf and Beszel's own
Authelia integration docs both require require_pkce/pkce_challenge_method,
which Authelia doesn't turn on by default.

immich.sh: _immich_offer_authelia_oidc() is real server-side automation,
not just paste-in instructions — confirmed the exact system-config "oauth"
JSON field names against Immich's own config-file.md and source (not
guessed, closing out the "needs one more verification pass" note this
repo's own CLAUDE.md already had on file). GET/PUT exchange the whole
config object, so it round-trips everything else unchanged. Needs an
admin API key that doesn't exist until first web-UI visit, so it's wired
into both the fresh-install path and the "update" rerun path.

audiobookshelf.sh, beszel.sh: both apps' OIDC config is UI-only (checked
against audiobookshelf.org and beszel.dev directly — no config API or env
var for the provider fields), so their new offers automate the Authelia
side and print exact paste-in values. Beszel also gets a real, separate
DISABLE_PASSWORD_AUTH/USER_CREATION toggle to fully replace its login,
gated behind a warning to register a working account first.

Also adds Audiobookshelf and Beszel as presets in authelia.sh's own
generic "Register another app" menu, and updates CLAUDE.md's OIDC
verification table to match reality (Immich now wired, Audiobookshelf
was wrongly listed as "high-confidence no", Beszel added).

services/gitea.sh

@@ -214,7 +214,7 @@ _gitea_offer_authelia_sso() {
     prompt_yn "  Require two-factor for Gitea logins via Authelia too? (y/n):" "y" _2fa
     [[ "$_2fa" =~ ^[Yy]$ ]] || AUTH_POLICY="one_factor"
 
-    if ! _authelia_provision_oidc_client "Gitea" "gitea" "$AUTH_POLICY" "y" \
+    if ! _authelia_provision_oidc_client "Gitea" "gitea" "$AUTH_POLICY" "y" "n" \
         "https://${GITEA_OIDC_DOMAIN}/user/oauth2/authelia/callback"; then
         log_warning "Couldn't register Gitea as an OIDC client in Authelia — skipping SSO setup."
         return 0