Leverage `updateMin` when syncing

[opatry]

• Store last sync (persist to avoid full reload on restart, but could begin with "in memory" storage to begin with and already benefit from that between 2 auto-refresh).
• Use it in list API calls (for task lists & tasks)
• Confirm we can then ignore showHidden in this case (might not concern deleted tasks but also completed, check carefully)
• Change the "delete stale tasks" strategy
  • Need the full picture of remote side to determine if a task or list stored in DB still has its remote counter part, not compatible with updateMin
  • We could do a purge only once per day or once per "session"/"launch", but anyway in a dedicated API, not part of the sync called in periodic auto-refresh