refactor: replace blocking std::fs and TCP I/O with async tokio equivalents

- R6: Convert 6 std::fs calls in session.rs to tokio::fs async
- R7: Convert std::fs::read_to_string in docker_packages.rs to async
- R8: Convert 3 std::fs calls in port_allocator.rs to async, switch to tokio::sync::Mutex
- R9+R10+R11: Fix blocking I/O in node_message.rs and nostr_discovery.rs
- R12: Convert electrs_status.rs from sync TCP to async tokio::net with 5s timeouts
- R4+R5: Spawn periodic cleanup tasks for endpoint and login rate limiters

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/nostr_discovery.rs

@@ -51,12 +51,9 @@ async fn load_or_create_nostr_keys(identity_dir: &Path) -> Result<Keys> {
         #[cfg(unix)]
         {
             use std::os::unix::fs::PermissionsExt;
-            tokio::task::spawn_blocking(move || {
-                std::fs::set_permissions(secret_path, std::fs::Permissions::from_mode(0o600))
-            })
-            .await
-            .context("spawn_blocking")?
-            .context("Failed to set Nostr key permissions")?;
+            fs::set_permissions(&secret_path, std::fs::Permissions::from_mode(0o600))
+                .await
+                .context("Failed to set Nostr key permissions")?;
         }
         fs::write(&pub_path, keys.public_key().to_hex())
             .await