Fix Last Changed updates during ePub rebuilds

Update Last Changed only when Bindery stores a new chapter or adds a new
chapter revision. Manual ePub generation, rebuilds, metadata edits and
rule changes leave it unchanged.

Save the timestamp in the same transaction as the chapter or revision.
This preserves the date if a later build fails and prevents retries from
moving it forward without another chapter change.

Add regression tests for new chapters, revisions, forced rebuilds, failed
builds, retries and transaction rollback. Adjust OPDS test data setup and
document what Last Changed records.

tests/test_opds.py

@@ -48,10 +48,10 @@ def add_book(title="A book", author="Writer", changed=None, url=None, included=T
     store = orchestrator.store
     url = url or f"https://example.com/books/{len(store.list_stories()) + 1}"
     sid = store.create_story(url, "example.com", title, author, [])
-    store.update_story(sid, last_changed=changed, site_values=meta or {})
     if included:
         html = "<p>Stored text.</p>"
         store.add_chapter(sid, url + "/1", "One", html, diffing.text_hash(html), "site", 0)
+    store.update_story(sid, last_changed=changed, site_values=meta or {})
     return sid