Fix scheduler serve_logs subprocess file descriptor errors

[kaxil]

Final blocker for 3.1.0b1

Fix scheduler serve_logs subprocess file descriptor errors

Remove workers parameter from uvicorn.run() in serve_logs to fix file
descriptor errors when scheduler starts serve_logs as a subprocess.
The original implementation used workers=2 (copied from Gunicorn) but
this caused multiprocessing issues in containerized environments.

Also implemented lazy app loading via get_app() function for better
initialization order and architectural consistency with main API.

The primary fix addresses: OSError: [Errno 9] Bad file descriptor. Secondary improvement ensures proper initialization timing.

This regression was introduced in #52581 when serve_logs was refactored from Flask to FastAPI.

---

Without this fix, we see following error when running airflow standalone fresh in an isolated container:

Error 1:

scheduler  | Traceback (most recent call last):
scheduler  | File "/.venv/bin/airflow", line 10, in <module>
scheduler  | sys.exit(main())
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/__main__.py", line 55, in main
scheduler  | args.func(args)
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/cli_config.py", line 49, in command
scheduler  | return func(*args, **kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/cli.py", line 114, in wrapper
scheduler  | return f(*args, **kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/providers_configuration_loader.py", line 54, in wrapped_function
scheduler  | return func(*args, **kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/commands/scheduler_command.py", line 52, in scheduler
scheduler  | run_command_with_daemon_option(
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/commands/daemon_utils.py", line 86, in run_command_with_daemon_option
scheduler  | callback()
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/commands/scheduler_command.py", line 55, in <lambda>
scheduler  | callback=lambda: _run_scheduler_job(args),
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/commands/scheduler_command.py", line 42, in _run_scheduler_job
scheduler  | with _serve_logs(args.skip_serve_logs), _serve_health_check(enable_health_check):
scheduler  | File "/usr/local/lib/python3.10/contextlib.py", line 135, in __enter__
scheduler  | return next(self.gen)
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/cli/commands/scheduler_command.py", line 62, in _serve_logs
scheduler  | from airflow.utils.serve_logs import serve_logs
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/serve_logs/__init__.py", line 20, in <module>
scheduler  | from airflow.utils.serve_logs.log_server import create_app
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/serve_logs/log_server.py", line 160, in <module>
scheduler  | app = create_app()
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/serve_logs/log_server.py", line 153, in create_app
scheduler  | JWTAuthStaticFiles(directory=log_directory, html=False),
scheduler  | File "/.venv/lib/python3.10/site-packages/airflow/utils/serve_logs/log_server.py", line 49, in __init__
scheduler  | super().__init__(*args, **kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/starlette/staticfiles.py", line 56, in __init__
scheduler  | raise RuntimeError(f"Directory '{directory}' does not exist")
scheduler  | RuntimeError: Directory '/root/airflow/logs' does not exist

Error 2:

This was because we had harcoded 2 workers! but this isn't needed

scheduler  | Process SpawnProcess-1:53:
scheduler  | Traceback (most recent call last):
scheduler  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
scheduler  | self.run()
scheduler  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 108, in run
scheduler  | self._target(*self._args, **self._kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/uvicorn/_subprocess.py", line 73, in subprocess_started
scheduler  | sys.stdin = os.fdopen(stdin_fileno)  # pragma: full coverage
scheduler  | File "/usr/local/lib/python3.10/os.py", line 1030, in fdopen
scheduler  | return io.open(fd, mode, buffering, encoding, *args, **kwargs)
scheduler  | OSError: [Errno 9] Bad file descriptor
scheduler  | Process SpawnProcess-1:54:
scheduler  | Traceback (most recent call last):
scheduler  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
scheduler  | self.run()
scheduler  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 108, in run
scheduler  | self._target(*self._args, **self._kwargs)
scheduler  | File "/.venv/lib/python3.10/site-packages/uvicorn/_subprocess.py", line 73, in subprocess_started
scheduler  | sys.stdin = os.fdopen(stdin_fileno)  # pragma: full coverage
scheduler  | File "/usr/local/lib/python3.10/os.py", line 1030, in fdopen
scheduler  | return io.open(fd, mode, buffering, encoding, *args, **kwargs)
scheduler  | OSError: [Errno 9] Bad file descriptor
triggerer  | Process SpawnProcess-1:53:
triggerer  | Traceback (most recent call last):
triggerer  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
triggerer  | self.run()
triggerer  | File "/usr/local/lib/python3.10/multiprocessing/process.py", line 108, in run
triggerer  | self._target(*self._args, **self._kwargs)
triggerer  | File "/.venv/lib/python3.10/site-packages/uvicorn/_subprocess.py", line 73, in subprocess_started
triggerer  | sys.stdin = os.fdopen(stdin_fileno)  # pragma: full coverage
triggerer  | File "/usr/local/lib/python3.10/os.py", line 1030, in fdopen
triggerer  | return io.open(fd, mode, buffering, encoding, *args, **kwargs)
triggerer  | OSError: [Errno 9] Bad file descriptor