refactor: centralize constants, eliminate unwraps, remove dead code, resolve TODOs

- R13+R16: Replace .expect() with .context()? in main.rs and identity.rs
- R17+R18+R19: Fix unwrap() calls in helpers and js-engine
- R20+R21: Remove #[allow(dead_code)] annotations and delete truly dead code
- R22-R26: Create constants.rs module, replace 21 hardcoded values across 12 files
- R28+R29: LND/DWN timeouts already present — verified
- R30-R33: Remove TODO comments, implement marketplace payment check

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

core/archipelago/src/main.rs

@@ -1,7 +1,7 @@
 // Archipelago Bitcoin Node OS - Native Backend
 // Pure Archipelago implementation, no StartOS dependencies
 
-use anyhow::Result;
+use anyhow::{Context, Result};
 use std::net::SocketAddr;
 use tracing::info;
 use tokio::signal;
@@ -9,6 +9,7 @@ use tokio::signal;
 mod api;
 mod auth;
 mod backup;
+mod constants;
 mod bitcoin_rpc;
 mod config;
 mod content_server;
@@ -122,7 +123,7 @@ async fn main() -> Result<()> {
     // Start server
     let addr: SocketAddr = format!("{}:{}", config.bind_host, config.bind_port)
         .parse()
-        .expect("Invalid bind address");
+        .context("Invalid bind address")?;
 
     // Spawn background update scheduler
     let update_data_dir = config.data_dir.clone();
@@ -155,9 +156,9 @@ async fn main() -> Result<()> {
     });
 
     // Graceful shutdown: wait for SIGTERM or SIGINT
-    let shutdown = async {
-        let mut sigterm = signal::unix::signal(signal::unix::SignalKind::terminate())
-            .expect("Failed to register SIGTERM handler");
+    let mut sigterm = signal::unix::signal(signal::unix::SignalKind::terminate())
+        .context("Failed to register SIGTERM handler")?;
+    let shutdown = async move {
         tokio::select! {
             _ = signal::ctrl_c() => {
                 info!("Received SIGINT (Ctrl+C), initiating graceful shutdown...");