Migrate preview reads to FastAPI

[cbusillo]

Summary

• migrate preview record and preview history reads to native FastAPI routes
• delete the legacy WSGI preview.read route matching/handler branch
• document preview read retirement and add regression coverage for stored-context auth before history listing

Validation

• uv run python -m unittest (2648 tests)
• uv run --extra dev ruff check --diff .
• uv run --extra dev ruff check .
• uv run --extra dev ruff check control_plane/http_app.py control_plane/service.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 tests/test_http_app.py tests/test_service.py
• uv run --extra dev mypy control_plane tests
• uv run /Users/cbusillo/Developer/codex-skills/jetbrains-inspection/scripts/jb-inspect.py closeout --repo "$PWD" --scope changed_files

Review

• Scout agents confirmed preview record/history are a safe single v2 slice.
• First review pass found history generation listing before authz; fixed before commit.
• Second review pass: 2/2 agents reported no blocking findings and merge-ready.

Notes

• Full ruff format --check . still reports existing repo-wide formatting drift outside touched files; touched files are formatted.

Refs #1322