style(meta/variants): normalize YAML header and comment indentation

Add the missing `---` document-start marker to two `meta/variants.yml`
files and re-indent the inline comment in web-app-mattermost so it
aligns with its enclosing block, matching the convention applied across
the other variants files in 5cfb24277.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

roles/web-app-keycloak/meta/variants.yml

@@ -1,3 +1,4 @@
+---
 # Matrix-deploy variant overrides for this role.
 # See docs/contributing/artefact/files/role/variants.md for the file format.
 

roles/web-app-mattermost/meta/variants.yml

@@ -1,3 +1,4 @@
+---
 # Matrix-deploy variant overrides for web-app-mattermost.
 # See docs/contributing/artefact/files/role/variants.md for the file format.
 
@@ -25,8 +26,8 @@
       enabled: true
       shared: true
 
-# variant pinning every dynamic services.yml flag to false so the
-# matrix also exercises the out-of-group_names branch.
+    # variant pinning every dynamic services.yml flag to false so the
+    # matrix also exercises the out-of-group_names branch.
     css:
       enabled: true
       shared: true