Send a regular email digest

wallabag should be able to send a regular (daily, weekly, monthly) digest of random/somehow selected articles to read.

Requires Cron and/or jobqueue.

The issue names no files, tests, or entry points; start by locating the existing cron or job-queue support, article-selection logic, and email delivery paths. Define how daily, weekly, and monthly schedules and random or selected articles should work, then verify that each schedule can send the expected digest.

Written by the indexing model from the issue text.