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/api/rpc/bitcoin.rs

@@ -86,7 +86,7 @@ impl RpcHandler {
         });
 
         let resp = client
-            .post("http://127.0.0.1:8332/")
+            .post(crate::constants::BITCOIN_RPC_URL)
             .basic_auth(&rpc_user, Some(&rpc_pass))
             .json(&body)
             .send()