fix: harden ElectrumX status — cached backend, stable frontend

Backend: cache status in RwLock, refresh every 15s via background task.
Eliminates per-request TCP race to ElectrumX that caused volatile errors.
Fix error classification so "Failed to read" is transient, not hard error.

Frontend: keep last-known-good data across failed polls, persist Tor
onion once discovered, adaptive polling (5s active / 30s synced).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/main.rs

@@ -139,6 +139,9 @@ async fn main() -> Result<()> {
     // Spawn disk space monitor (warns at 85%, auto-cleans at 90%)
     disk_monitor::spawn_disk_monitor(config.data_dir.clone());
 
+    // Spawn ElectrumX status cache (refreshes every 15s, serves cached data to avoid race conditions)
+    electrs_status::spawn_status_cache();
+
     let startup_ms = startup_start.elapsed().as_millis();
     info!("Server listening on http://{} (startup: {}ms)", addr, startup_ms);
     info!("RPC API: http://{}/rpc/v1", addr);