Add an OPDS catalog

A reader app can browse the stored books by title, author and last
change, search them, and download them. The download and cover
routes share their code with the story page.

An admin turns the catalog on in the settings. The login follows
AUTH_METHOD: none needs no login, local and cwa take the account
login, and proxy takes the user name from the proxy with an OPDS
password that each user sets. The API key does not open the catalog.

tests/test_api.py

@@ -472,6 +472,9 @@ def test_settings(client, monkeypatch, tmp_path):
         "pause_uploads": False,
         "ingest_available": True,
         "personal_ini": "[defaults]\nis_adult:true\n",
+        "opds_enabled": False,
+        "opds_password_set": False,
+        "opds_user": None,
         "download_template": "{title_sort} - {authors}",
         "download_date_format": "%b, %Y",
         "story_jobs_hours": 36,