Version cut 1.9.3: close the panel-redesign docs prep, bump the pin

CHANGELOG.md audited against the full commit history from the repo's
first commit (8ad56ed) through HEAD: version/date headings, entry
completeness, and technical claims against their diffs all check out,
no discrepancies found.

The Unreleased section (panel-redesign mockups, contract, and
decisions; preflight plan) is docs-only prep — development.md's own
rule would hold the cut until a product stage ships — but the owner
asked for a version cut now, so it becomes 1.9.3 (PATCH) and a fresh
Unreleased opens. Pin bumped in deploy/docker-compose.yml, README's
quickstart, guide.md, and development.md's planned-cuts table, which
also gains the new row (renumbering inbound-antispam-panel to 11).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -21,7 +21,7 @@
 
 services:
   selfpost:
-    image: ghcr.io/mixeme/selfpost:1.9.2
+    image: ghcr.io/mixeme/selfpost:1.9.3
     restart: unless-stopped
     environment:
       SELFPOST_HOSTNAME: "${SELFPOST_HOSTNAME:?set the mail/panel hostname, e.g. mail.example.com}"