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/constants.rs

@@ -0,0 +1,19 @@
+/// Centralized constants for the Archipelago backend.
+/// Avoids hardcoded values scattered across the codebase.
+
+/// Bitcoin Core RPC endpoint (localhost only).
+pub const BITCOIN_RPC_URL: &str = "http://127.0.0.1:8332/";
+
+/// DWN (Decentralized Web Node) health check endpoint.
+pub const DWN_HEALTH_URL: &str = "http://127.0.0.1:3100/health";
+
+/// Tor SOCKS5 proxy for outbound onion connections.
+pub const TOR_SOCKS_PROXY: &str = "socks5h://127.0.0.1:9050";
+
+/// DNS-over-HTTPS providers for secure DNS resolution.
+pub const DNS_PROVIDERS: &[&str] = &[
+    "https://cloudflare-dns.com/dns-query",
+    "https://dns.google/dns-query",
+    "https://dns.quad9.net/dns-query",
+    "https://dns.mullvad.net/dns-query",
+];