ci: call the shared checks.yml instead of the local copy

[lesnik512]

Summary

Part of modern-python/.github#95. ci.yml and scheduled.yml call the shared checks.yml and the local _checks.yml goes. This is the first caller, chosen because it needs a service container: it exercises the workflow from the shared-checks branch ref in modern-python/.github before that PR merges, and proves the postgres inputs path.

Draft until modern-python/.github#101 merges; the ref then flips to @main.

Changes

• Postgres comes from the service-* inputs, its password via -e in service-options; DB_DSN from test-env; DISABLE_SQLALCHEMY_CEXT_RUNTIME from free-threaded-env, so it applies on 3.14t only, as before.
• The matrix is derived from requires-python now, so lint runs on 3.11 (the floor) where the local copy pinned 3.13, and pytest runs just install where the local copy ran uv sync --all-extras --no-install-project. Both are what the standard says; if either fails here it is a finding about this repo.
• PYTHONDONTWRITEBYTECODE and PYTHONUNBUFFERED are dropped; they only affected log buffering.

Checklist

• CI green from the branch ref
• Ref flipped to @main after feat(ci): add the shared checks.yml with a derived Python matrix .github#101 merges

[lesnik512]

Closed with modern-python/.github#95: the org is staying on per-repo _checks.yml.