Bound daemon startup and shutdown and exit with a useful code

start_daemon and stop_daemon polled "while True" with no bound, and no
seaf-cli exit code was checked. A daemon that failed to start left the
container spinning forever with nothing pointing at the cause, and there was
no signal handling, so "docker stop" waited the full 10 second grace period
and then SIGKILLed the process with the daemon still running.

Readiness polling is now bounded by DAEMON_START_TIMEOUT and
DAEMON_STOP_TIMEOUT. The stop timeout is 8 seconds so a graceful shutdown
fits inside the docker stop grace period, and a test asserts that relation.

SIGTERM and SIGINT raise GracefulShutdown, and the lifecycle moved into
start.run(), which always stops the daemon on the way out and returns the
process exit code: 0 on a clean finish or a stop signal, 1 on a daemon or
configuration failure.

seaf-cli exit codes are checked. A failed sync is reported per library and
does not stop the remaining libraries, since one bad library should not take
the container down.

Verified in the built image: "docker stop" now returns in 0s with exit code 0
and the daemon stopped, against 10s and exit 137 before. A daemon that cannot
start exits 1 with the failing command and its exit code in the log.

Also bumps actions/setup-python to v7, which clears the last Node 20
deprecation warning; v7 only drops the unused pip-install input.

IMPROVEMENTS.md

@@ -90,13 +90,14 @@ Acceptance criteria:
 
 ### Make startup and shutdown deterministic
 
-- [ ] Check every important subprocess return code.
-- [ ] Add timeouts to daemon startup, shutdown, and readiness polling.
-  Confirmed: `start_daemon` and `stop_daemon` poll `while True` with no bound,
-  so a daemon that never changes state hangs the container silently.
-- [ ] Handle `SIGTERM` and `SIGINT` and stop the daemon in `finally`.
-- [ ] Return a nonzero container exit code on configuration or daemon failure.
-- [ ] Produce actionable errors instead of waiting forever.
+- [x] Check every important subprocess return code. A failed `seaf-cli sync` is
+  reported per library and does not stop the remaining ones.
+- [x] Add timeouts to daemon startup, shutdown, and readiness polling. See
+  `DAEMON_START_TIMEOUT` and `DAEMON_STOP_TIMEOUT` in `dsc/const.py`; the stop
+  timeout is deliberately below the 10 second `docker stop` grace period.
+- [x] Handle `SIGTERM` and `SIGINT` and stop the daemon in `finally`.
+- [x] Return a nonzero container exit code on configuration or daemon failure.
+- [x] Produce actionable errors instead of waiting forever.
 
 Acceptance criteria:
 
@@ -113,7 +114,9 @@ Acceptance criteria:
 - [ ] Add regression tests for externally reported password and deletion
   threshold issues.
 - [ ] Add a container smoke test for imports, installed CLI version, startup,
-  and shutdown using controlled fakes.
+  and shutdown using controlled fakes. Graceful shutdown was verified by hand
+  in the built image (`docker stop` returns immediately with exit code 0
+  instead of being killed after 10 seconds); it is not yet automated in CI.
 - [x] Keep image construction as a separate CI job so packaging failures are
   distinguishable from application failures. The `unit-test` job does not
   depend on the image build.