Skip importing the app in the parent process when using subprocesses

[Kludex]

Stacked on #2987.

Summary

This is the actual fix for #2980. Since #2919, uvicorn.run() eagerly imports the app in the parent process - but with --workers or --reload the parent only supervises subprocesses, which import the app themselves. The parent's copy is pure overhead. With #2987 the worker already builds its own Server and reports readiness, so the parent no longer needs the app at all in the subprocess case.

The change is two lines: only call config.load_app() in the parent when it actually serves the app in-process.

if not config.use_subprocess:
    config.load_app()

Thread workers (#2900) stay on the eager path: they aren't subprocesses, so use_subprocess is false and the parent loads as before, priming sys.modules for them.

Measured

200 MB app, workers=2, Python 3.14:

	supervisor RSS
#2987 (base)	245 MB
this PR	44 MB

Workers unchanged (~247 MB each).

Behavior change

A bad import string with --workers is no longer caught by the parent's eager load; it now surfaces when the worker fails to boot and the supervisor shuts down with STARTUP_FAILURE (the mechanism added in #2987, covered by test_multiprocess_stops_on_startup_failure). So #2440 fail-fast is preserved, just relocated from parent to supervisor. Under --reload, a broken app logs the error and waits for a file change instead of killing the reloader.

The single-process path is untouched: it still imports eagerly before the event loop opens, keeping #941 fixed (test_run_imports_app_before_starting_event_loop).

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}