Allow users to configure how long items stay in the "Leaving Soon" state
before deletion, replacing vague "next cleanup cycle" text with a concrete
date in notifications and Plex collection descriptions.

The duration field accepts formats like "7d" (days) or "24h" (hours).
When set, all notification providers (email, Discord, Slack, Telegram)
and the Plex collection summary display the exact removal date.
Falls back to schedule-derived date if duration is not configured.

Items added to the leaving soon collection were deleted on the very
next run regardless of the configured duration. The duration property
was display-only — used for notifications and collection summaries but
never checked before deleting.

Add a state file (/config/.deleterr_state.json) that records when each
item was tagged to the leaving soon collection. On each run, items are
only eligible for deletion once tagged_at + duration has elapsed.

Key behaviors:
- No duration configured: existing behavior (delete on next run)
- Duration not elapsed: skip deletion, log remaining time
- Duration elapsed: delete as normal
- State file missing/corrupt: treat items as newly tagged (conservative)
- Daily schedule with 7d duration: items wait the full 7 days across
  multiple runs, staying in the collection until eligible

Fixes #244