[High] Mesh relay requires app in foreground (esp. iOS) #5

Summary

Background BLE relaying is not implemented. iOS suspends BLE aggressively; the app documents that peers must keep the app foregrounded.

Why it matters

The courier / pocket-relay story (walk a message out of a jam bubble) fails if phones in pockets do not relay. Dense meshes shrink to whoever has the screen on.

Evidence

• modules/ble-mesh/README.md: no UIBackgroundModes; background mesh is open problem [Critical] No forward secrecy for sealed messages #4.
• docs/THREAT-MODEL.md open problem: “iOS background operation.”
• README manual test instructions: keep the app in the foreground on both phones.

Proposed fix

Measure real CoreBluetooth / Android background behavior on current OS versions; implement best-effort background advertising/scanning/restoration where possible; document residual limits honestly in-app and in the threat model.