release: 1.2.0

Close CHANGELOG [Unreleased] as 1.2.0; correct missing entries; bump the
compose pin and image references.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -95,7 +95,7 @@ docker run --rm -d --name selfpost-try \
   -e SELFPOST_HOSTNAME=mail.local.test \
   -e PANEL_COOKIE_SECURE=false \
   -v selfpost-try-data:/data \
-  ghcr.io/mixeme/selfpost:1.1.0
+  ghcr.io/mixeme/selfpost:1.2.0
 ```
 
 **Get the setup URL** (pick one):