feat(gateway): wire GATEWAY_HONEYTOKEN through the deploy

The planted honeytoken bearer trap already existed in the gateway + compose, but
no workflow fed GATEWAY_HONEYTOKEN, so it was always empty (inert). Wire the
per-contour TEST_/PROD_GATEWAY_HONEYTOKEN secret into the ci deploy, prod-deploy
and prod-rollback env (the prod path via the shared deploy/write-prod-env.sh),
document it (deploy/README + .env.example), and fix the stale compose comment.

Empty secret = trap off (no ":?" guard), so a deploy is safe before the operator
sets the value + plants the bait. On prod (IP ban on) presenting it earns a 24h
ban + alarm; on test (ban off) it logs + a ban metric.

GF_SMTP_ENABLED is enabled separately via the TEST_/PROD_GF_SMTP_ENABLED Gitea
variables (=true) — no code change.

deploy/.env.example

@@ -108,3 +108,10 @@ GATEWAY_VK_APP_SECRET=
 # the confidential OAuth 2.1 code exchange under this protected key. The app id + redirect URL
 # come from VITE_VK_APP_ID / VITE_VK_ID_REDIRECT_URL above. All three empty disables link.vk.*.
 GATEWAY_VK_ID_CLIENT_SECRET=
+
+# --- Gateway anti-abuse ------------------------------------------------------
+# Planted honeytoken bearer value: any request presenting it is flagged — a 24h IP ban
+# where the IP ban is on (prod), logs + a ban metric otherwise (test). Plant the value
+# somewhere an attacker would find it; empty disables the trap. Gitea
+# TEST_/PROD_GATEWAY_HONEYTOKEN (secret).
+GATEWAY_HONEYTOKEN=