Restrict production wp-admin/wp-login to an IP allowlist (opt-in)

New WP_ADMIN_ALLOWED_IPS env var (space-separated CIDR ranges, matching
Caddy's remote_ip syntax directly). Defaults to 0.0.0.0/0 ::/0 (today's
open behavior) when unset, resolved at the Compose level rather than in
the Caddyfile so an unset var can never leave `remote_ip` with zero
arguments and fail Caddy's startup. admin-ajax.php stays exempt since
WooCommerce routes logged-out storefront AJAX through it. Production
only — staging is already LAN-only.

Verified live: a scratch Caddy instance on a Docker network, with a
sibling container's actual observed source IP used to prove both the
blocked (403 on wp-admin/wp-login, everything else unaffected) and
allowed (matching CIDR lets it through) cases, plus `docker compose
config` to confirm the unset-var default resolves correctly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MqhgnCRjjVLFnZ3Jpn93Vg

.env.example

@@ -61,3 +61,10 @@ MAILERLITE_API_KEY=
 # --- Hardcover (AO3-style tag browsing — genre/mood/content-warning/tag sync) ---
 # Get a token: hardcover.app account settings -> Hardcover API -> New API Key
 HARDCOVER_API_TOKEN=
+
+# --- Admin access restriction (production only) ---
+# Space-separated IPs/CIDR ranges allowed to reach /wp-admin and
+# /wp-login.php (matches Caddy's `remote_ip` syntax directly — no quotes,
+# space-separated even though most other values here are one token).
+# Leave blank to leave wp-admin reachable from anywhere (today's behavior).
+WP_ADMIN_ALLOWED_IPS=