feat: add --run-as-service for background daemon mode (linux/macos only)

[mforce]

Summary

Adds a --run-as-service flag (and friends) to thoth so the app can run detached from the terminal — closing the shell no longer shuts it down. Implemented as a small service module that handles POSIX daemonization, PID-file management, and systemd unit generation.

New flags

Flag	Purpose
--run-as-service	Double-fork to detach. Writes PID to $THOTH_DATA_DIR/service.pid, redirects stdout/stderr to $THOTH_DATA_DIR/service.log. Forces --server --no-tray --no-open --no-splash.
--service-stop	SIGTERM the daemon (then SIGKILL after 10s).
--service-status	Print running/stopped + PID.
--service-restart	Stop existing daemon, then start a new one.
--install-systemd-service	Write ~/.config/systemd/user/thoth.service with Type=simple — recommended long-term setup.
--pid-file / --service-log	Override default paths.

Notes

• bin/thoth already forwards extra args (exec "$PYTHON" launcher.py "$@"), so no installer change was required.
• Windows is intentionally not supported by the hand-rolled daemon; on Windows the flag prints a message pointing users to Task Scheduler / NSSM. macOS works the same as Linux.
• Defaults respect THOTH_DATA_DIR (matches the existing convention in logging_config.py).

Usage

thoth --run-as-service        # start in background
thoth --service-status        # check
thoth --service-stop          # stop
thoth --install-systemd-service && \
  systemctl --user daemon-reload && \
  systemctl --user enable --now thoth.service

Test plan

• Unit tests in tests/test_service.py cover read/write/remove PID, is_alive, status messages, stop on stale PID, stop on a real child process, systemd unit generation, default-path resolution, and CLI routing for --service-status / --service-stop / --install-systemd-service.
• End-to-end manual smoke test: forked a real daemon via service.daemonize, verified the PID file is written, is_alive reports true, stdout is captured to the log file, and stop_service cleanly terminates and removes the PID file.
• Try on macOS (only Linux verified locally).
• Confirm --install-systemd-service produces a working unit when used end-to-end on a system with systemctl --user.

[siddsachar]

Thanks for this. The POSIX daemon approach looks promising: I verified tests/test_service.py in WSL and did a real service.daemonize smoke where the PID file/log/stop path worked.

Before merge, could you please address two platform-safety issues?

1. Windows/non-POSIX service management should be hard-gated before calling PID liveness logic. Locally on Windows, tests/test_service.py::test_is_alive_for_self_and_invalid raises KeyboardInterrupt because os.kill(pid, 0) is not a safe liveness probe there. Since launcher.py exposes --service-status / --service-stop on all platforms, those commands can currently reach unsafe logic even though daemon mode is Linux/macOS only.

2. --install-systemd-service should be Linux-only. Right now install_systemd_unit() rejects Windows but would write ~/.config/systemd/user/thoth.service on macOS, which is not useful and contradicts the systemd-only behavior.

I can take care of wiring tests/test_service.py into our CI/release focused test commands once those feature-level fixes are in.