fix(deploy): write .env to /opt/staging not \$HOME/staging

\$HOME in an unquoted heredoc expands on the runner (not the VPS), so the
VPS received the literal runner path (/root/staging/.env) which didn't exist.
Using the explicit /opt/staging/.env path (consistent with compose.yml and
mkdir) fixes the permission denied error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/deploy-staging.yml

@@ -213,8 +213,8 @@ jobs:
           # printf keeps every line indented (no column-0 content) so YAML stays valid.
           printf 'CLERK_SECRET_KEY=%s\nADMIN_CLERK_SECRET_KEY=%s\n' \
             "${CLERK_SECRET_KEY}" "${ADMIN_CLERK_SECRET_KEY}" \
-            > $HOME/staging/.env
-          chmod 600 $HOME/staging/.env
+            > /opt/staging/.env
+          chmod 600 /opt/staging/.env
 
           cd /opt/staging
           podman compose up -d --force-recreate --remove-orphans