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>

docs/guide.md

@@ -173,7 +173,7 @@ cat ./data/setup-token
 #### Fixed image tag
 
 `deploy/docker-compose.yml` pins an explicit version (`ghcr.io/mixeme/selfpost:X.Y.Z`),
-deliberately never `:latest`. The current pin is `1.9.2`. Intermediate
+deliberately never `:latest`. The current pin is `1.9.3`. Intermediate
 CHANGELOG sections (`0.2.0`…`0.6.0`) record development cuts from before that
 image was published. Pinning matters because of the backup version check (see
 [Full backup and restore](#full-backup-and-restore)): the panel binary's