Name downloaded books with a template

The new "Book Downloads" group in Settings holds a Calibre template for the file name of a
direct download. Without a template, the name stays the title. The name is also the title if
the template fails or gives no name for a book.

README.md

@@ -445,6 +445,53 @@ After the import, Bindery checks the site for new chapters and sends the book to
 "Check the site for new chapters after import", Bindery only rebuilds the book of a story that it
 downloaded before.
 
+## File names of downloaded books
+
+"Download ePub" on a story page sends the book to your browser with a file name. Without a template,
+the name is the title, for example `The Book.epub`.
+
+An admin can set a template for the name under "Book Downloads" in Settings. The template follows
+the rules of [Calibre's templates](https://manual.calibre-ebook.com/template_lang.html) for file
+names, so a template from Calibre gives the same name in Bindery. Bindery adds `.epub`. For example,
+`{series:|| - }{series_index:0>2s|| - }{title} - {authors}` gives
+`Saga, The - 02 - Book, The - Ann Writer.epub`.
+
+| Field | Value |
+| --- | --- |
+| `{title}` | The title. A leading "The", "A" or "An" moves to the end, as in Calibre: `Book, The`. |
+| `{title_sort}` | The same value as `{title}`. |
+| `{authors}`, `{author}` | The authors, with ` & ` between them. |
+| `{author_sort}` | The authors with the last name first: `Writer, Ann`. |
+| `{series}` | The series. A leading article moves to the end. |
+| `{series_index}` | The series number: `2`, or `2.5`. |
+| `{tags}` | The tags in sorted order, with `, ` between them. |
+| `{languages}` | The language, as Bindery stores it. |
+| `{publisher}` | The story site. |
+| `{pubdate}` | The published date, in the form `May, 2020`. |
+| `{timestamp}` | The date when Bindery added the story. |
+| `{last_modified}` | The date when the chapters last changed. |
+| `{id}` | The CWA book ID. For a story with no library book, and without CWA, the Bindery story ID. |
+
+- A field with no value becomes empty text. So does a field that Bindery does not have, such as
+  `{rating}` or a custom column.
+- `{field:format|text before|text after}` adds the two texts only when the field has a value. Each
+  of the three parts can be empty. `{series:|| - }` adds ` - ` after the series.
+- The format is a Python format. `{series_index:0>3s}` gives `002`, and `{title:.20s}` gives the
+  first 20 characters. `{author_sort[0]}` gives the first character.
+- `{{` and `}}` give braces. Bindery makes each run of spaces one space.
+- A download has one file name. If the template has folders, Bindery uses the part after the last
+  `/`. That is the name that Calibre gives the file.
+- Bindery does not support template functions, such as `{title:uppercase()}`, or the program modes
+  of Calibre. It refuses such a template when you save it.
+- If the template fails for a book, or gives an empty name, Bindery names the file after the title.
+  The log has a warning.
+- Bindery replaces each character that a file name cannot have with `_`. A name has 100 characters
+  and 200 bytes at most.
+
+Bindery follows Calibre 9.2.1 with its default tweaks, for English. The order of tags with letters
+outside ASCII can differ from Calibre. The template applies only to downloads in the browser. It
+does not change the names of the files in `INGEST_DIR`. The browser can ignore the name.
+
 ## Download modes
 
 | Mode | What it does |