Add opt-in extended analytics with slow background refresh

[thrr87]

Keep the app light by default.

Proposed behavior:

• collect and show only Usage remaining by default
• add one toggle for extra local analytics
• start extra collection only after the user turns it on
• import in small batches and show honest loading states
• keep the app responsive while data fills in
• refresh extra analytics less often than Usage remaining and never just because the menu opened
• keep data local and keep Usage remaining available while extra analytics loads

This is a later stage. First measure and release the lightweight SQLite core in #39, then set refresh intervals and batch sizes from real results. Do not add this toggle to #39.