Support multiple portal GitLab targets

[yoshifuminakamura]

Summary

• add named GitLab pipeline targets for Portal submit/dry-run
• keep the existing RESULT_SERVER_GITLAB_REPO / RESULT_SERVER_GITLAB_TOKEN single-target fallback
• add target selection to the admin execution profile form
• document RESULT_SERVER_GITLAB_TARGETS and target-specific token env names

Configuration

• single target: RESULT_SERVER_GITLAB_REPO + RESULT_SERVER_GITLAB_TOKEN
• multiple targets: RESULT_SERVER_GITLAB_TARGETS plus RESULT_SERVER_GITLAB_TOKEN_<TARGET_ID>
• named targets require their own token; they do not fall back to the global token

Validation

• /home/nakamura/fugakunext/venv/bin/python -m pytest result_server/tests/test_execution_profiles.py result_server/tests/test_app_dev_security.py result_server/tests/test_api_routes.py result_server/tests/test_preflight.py
• /home/nakamura/fugakunext/venv/bin/python result_server/tests/run_result_server_tests.py
• python3 -m py_compile result_server/routes/admin.py result_server/utils/gitlab_pipeline.py