Show an estimated page count for each built book

Bindery counts the pages as the last step of a build, from the text of the ePub. The story list and the OPDS catalog show the count. A story has no count until its book is built.

The method follows "Paragraphs (APNX Accurate)" of the Count Pages plugin for calibre. Bindery counts each paragraph that a reader sees, so the same text gives the same count as <p> elements, as <div> elements and as lines with <br> between them. An empty paragraph is not counted.

tests/test_opds.py

@@ -205,6 +205,17 @@ def test_entry_shows_the_book_metadata_and_cover(client):
     assert client.get(f"/opds/books/{add_book()}/cover").status_code == 404
 
 
+def test_entry_shows_the_page_count_of_a_built_book(client):
+    sid = add_book()
+    orchestrator.store.set_setting(sid, "page_count", 321)
+    add_book(title="Not built")
+    contents = {
+        e.findtext("a:title", namespaces=NS): e.findtext("a:content", namespaces=NS)
+        for e in entries(feed(client.get("/opds/title")))
+    }
+    assert contents == {"A book": "Chapters: 1\nPages: 321", "Not built": "Chapters: 1"}
+
+
 def test_cover_of_an_unsafe_type_is_left_out(client):
     sid = add_book()
     orchestrator.store.put_image(sid, "https://example.com/c.svg", "svg", "image/svg+xml", b"<svg/>", is_cover=True)