feat: add mix openapi.export task

[mandarini]

What this is

• A mix openapi.export task that materializes the OpenApiSpex-generated REST spec to priv/openapi.json, plus a PR-time drift gate.

What maintainers need to do

• Bootstrap once: mix openapi.export, commit priv/openapi.json.
• On future PRs touching lib/realtime_web/: re-run the task and commit the updated snapshot.

What it's used for

• Today the REST management spec only exists at runtime via /api/openapi. Committing a snapshot makes it a static, versioned artifact SDKs and type generators can pin to.
• Scope is REST only. The channel protocol (broadcast, presence, postgres_changes) is intentionally out of scope — would need AsyncAPI separately.

What was added and why

• lib/mix/tasks/openapi.export.ex — task that writes RealtimeWeb.ApiSpec.spec() JSON to priv/openapi.json (or --output PATH).
• .github/workflows/openapi.yml — re-runs the task on PRs touching lib/realtime_web/ and fails on git diff if the committed snapshot is stale. Uploads the regenerated spec as an artifact.