ci: add GitLab CI/CD pipeline with prod, staging, and MR preview environments

- main → production, staging branch → staging, MR → ephemeral preview
- All env config (URLs, Appwrite credentials) injected from GitLab CI/CD
  variables at build time — no .env files committed
- Preview environments auto-deploy per MR with URL {mr-iid}.{BASE_DOMAIN}
  and are torn down when the MR is closed
- Update .gitignore to exclude .env.production/.staging/.preview

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

.gitignore

@@ -5,6 +5,8 @@ dist-ssr
 .env
 .env.local
 .env.production
+.env.staging
+.env.preview
 .DS_Store
 *.pem
 coverage