Migrate product environment reads to FastAPI

[cbusillo]

Summary

• migrate core /v1/products product/site read-model routes from legacy WSGI to native FastAPI
• retire the direct WSGI product-environment read branches while keeping /v1/repo-product-mapping and /v1/agent/context on the retained fallback
• add native FastAPI contract, authz, redaction, fallback-precedence, and retirement coverage
• update service-boundary and compatibility-retirement docs

Local Validation

• uv run python -m unittest (2681 tests)
• uv run python -m unittest tests.test_http_app tests.test_service (706 tests)
• uv run python -m unittest tests.test_http_app.FastApiProductEnvironmentReadTests
• uv run --extra dev ruff check .
• 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
• git diff --check
• JetBrains changed-files closeout: clean, 0 problems

Review

• Scout/review agents green; final post-polish review found no blockers.

Note: whole-repo ruff format --check . still reports pre-existing unrelated formatting drift in 56 files; touched files are formatted.