Move merge train reads to FastAPI

[cbusillo]

Summary

• move merge-train admission, controller-status, and policy-target reads to native FastAPI routes
• retire the matching legacy WSGI read branches and keep direct WSGI calls fail-closed
• update FastAPI route coverage, WSGI retirement coverage, and v2 compatibility docs

Validation

• uv run python -m unittest tests.test_http_app.FastApiMergeTrainReadTests tests.test_service.LaunchplaneServiceTests.test_merge_train_reads_are_retired_from_legacy_wsgi_app
• uv run --extra dev ruff check .
• uv run --extra dev mypy control_plane tests
• uv run python -m unittest
• uv run /Users/cbusillo/Developer/codex-skills/jetbrains-inspection/scripts/jb-inspect.py closeout --repo "/Users/cbusillo/Developer/launchplane" --scope changed_files

Review

• Code review agents: green (2 useful green reviews; 1 empty/no-op response)