web-ui: clock indicator on sessions a cron is scheduled to post into (#500)

A session that a session-targeted cron will post into (for example a scheduled resume that promised "I'll report when it's green") looked completely idle: the background chip only reflected live background jobs and job-output watches. listSessions now attaches a per-session count of enabled, unarchived crons whose destination targets the session, and sessionBackground lists them (id, title, next fire time) for the inspector panel. The session list shows a clock indicator on those sessions so pending scheduled activity is visible at a glance.

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