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.
Jobs download new chapters, recheck chapters, replace the book, update
metadata or rebuild from stored chapters. Locked metadata fields keep
their values. A scheduler queues updates. An update finds the library
book of a story through CWA, and fails with an error if it cannot, so
that it never makes a duplicate book.
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