chore(mail): pinned-version update check off, Gmail postscreen whitelist, docs

- compose: ENABLE_UPDATE_CHECK=0 (image intentionally pinned to DMS v15.1.0).

- postscreen: permit Google SMTP outbound ranges (Gmail retries from rotating IPs,

  '450 PASS NEW' never completes) — keeps LinkedIn/Gmail mail out of the tarpit.

- README: DNS/ops notes; user-patches: whitelist copy path.

config/linkedin_whitelist.cidr

@@ -1,2 +1,12 @@
 108.174.0.0/16	PERMIT
 144.2.0.0/15	PERMIT
+# Google SMTP outbound (Gmail retries from rotating 74.125/209.85/...; postscreen 450 PASS NEW never completes)
+74.125.0.0/16	PERMIT
+209.85.128.0/17	PERMIT
+64.233.160.0/19	PERMIT
+66.102.0.0/20	PERMIT
+66.249.80.0/20	PERMIT
+72.14.192.0/18	PERMIT
+173.194.0.0/16	PERMIT
+207.126.144.0/20	PERMIT
+216.239.32.0/19	PERMIT