An error outside a job, such as a full disk, stopped the thread, and no
job ran until a restart. The health check now answers 503 when a job
thread stopped.
A key of the chapters, rules and metadata of a story tells if its book
changed. A check without a change no longer builds the book again, also
for a book that waits for an upload. A held book goes out without a new
build. The Rebuild button still builds and sends in each case.
tim