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.

tests/test_optional_cwa.py

@@ -137,6 +137,15 @@ def test_cwa_off_shows_in_the_config_and_refuses_cwa_logins(client, monkeypatch,
     assert response.json["error"].startswith("AUTH_METHOD=cwa needs CWA, but CWA_ENABLED is false.")
 
 
+def test_without_cwa_the_download_template_uses_the_story_id(client, cwa_off):
+    assert client.get("/api/config").json["download_fields"]["id"] == "The Bindery story ID"
+    story_id = add_story(calibre_id=77)
+    client.put("/api/settings", json={"download_template": "{title} {id}"}, headers=H)
+    response = client.get(f"/api/stories/{story_id}/epub")
+    response.close()
+    assert f'filename="My Title {story_id}.epub"' in response.headers["Content-Disposition"]
+
+
 def test_link_setting_defaults_on_and_persists(client, monkeypatch, tmp_path, library):
     assert client.get("/api/settings").json["link_books"] is True
     response = client.put("/api/settings", json={"link_books": False}, headers=H)