Move work graph reads to FastAPI

[cbusillo]

Summary

• Move GET /v1/work-graph/snapshot and GET /v1/work-graph/github/issues to native FastAPI routes.
• Retire the matching legacy WSGI read-route branches and WSGI-only work-graph GET helpers.
• Preserve POST rank/reconcile on the mounted WSGI fallback and update docs/tests for the new ownership boundary.

Verification

• uv run python -m unittest tests.test_http_app.FastApiProductEnvironmentReadTests tests.test_service.LaunchplaneServiceTests.test_work_graph_reads_are_retired_from_legacy_wsgi_app
• uv run --extra dev ruff check control_plane/http_app.py control_plane/service.py control_plane/work_graph_http.py tests/test_http_app.py tests/test_service.py
• uv run --extra dev ruff format --check control_plane/http_app.py control_plane/service.py control_plane/work_graph_http.py tests/test_http_app.py tests/test_service.py
• uv run --extra dev mypy control_plane/http_app.py control_plane/service.py control_plane/work_graph_http.py tests/test_http_app.py tests/test_service.py
• git diff --check
• uv run --extra dev ruff check .
• uv run --extra dev mypy control_plane tests
• uv run python -m unittest (2748 tests)
• JetBrains changed-files closeout: clean

Repo-wide uv run --extra dev ruff format --check . still reports pre-existing formatting drift in untouched files; changed files are formatted cleanly.

Review

• Multi-agent review batch green after one low-severity stale WSGI wiring cleanup.
• Final post-cleanup review green.