docs(app): how scope discovery works under the hood

[MrAlders0n]

Replaces the How Scope Discovery Works section of app_how_it_works.md with the reviewed write-up of how scope discovery works under the hood:

• When it runs (Passive/Hybrid only, sweep timing)
• Who controls it (server flag, admin lock, 7-day min, firmware v1.16.0+)
• Not asking twice (server + phone both say due)
• Sharing the radio (one pending request, the 4 s lease, Hybrid TX up to ~4 s late)
• Zero-hop route borrow and its accepted edge cases
• Answer timeouts, no-answer backoff, and the v1.16.0 full contact list limit

Text is added exactly as reviewed; only blank lines were added so each paragraph and the list render correctly.

Stacked on #30 (both touch this page). Merge #30 first.

• mkdocs build --strict passes