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/helpers/src/lib.rs

@@ -164,12 +164,16 @@ impl AtomicFile {
 impl std::ops::Deref for AtomicFile {
     type Target = File;
     fn deref(&self) -> &Self::Target {
-        self.file.as_ref().unwrap()
+        self.file
+            .as_ref()
+            .expect("AtomicFile already consumed by save() or rollback()")
     }
 }
 impl std::ops::DerefMut for AtomicFile {
     fn deref_mut(&mut self) -> &mut Self::Target {
-        self.file.as_mut().unwrap()
+        self.file
+            .as_mut()
+            .expect("AtomicFile already consumed by save() or rollback()")
     }
 }
 impl Drop for AtomicFile {
@@ -177,7 +181,11 @@ impl Drop for AtomicFile {
         if let Some(file) = self.file.take() {
             drop(file);
             let path = std::mem::take(&mut self.tmp_path);
-            tokio::spawn(async move { tokio::fs::remove_file(path).await.unwrap() });
+            tokio::spawn(async move {
+                if let Err(e) = tokio::fs::remove_file(&path).await {
+                    tracing::warn!("failed to remove tmp file {}: {}", path.display(), e);
+                }
+            });
         }
     }
 }
@@ -230,7 +238,13 @@ impl<T: 'static + Send> TimedResource<T> {
 
     pub async fn get(self) -> Option<T> {
         let _ = self.ready.send(());
-        self.handle.await.unwrap()
+        match self.handle.await {
+            Ok(val) => val,
+            Err(e) => {
+                tracing::error!("TimedResource task panicked: {}", e);
+                None
+            }
+        }
     }
 
     pub fn is_timed_out(&self) -> bool {
@@ -250,7 +264,7 @@ pub async fn spawn_local<
         tokio::runtime::Builder::new_current_thread()
             .enable_all()
             .build()
-            .unwrap()
+            .expect("failed to build tokio current-thread runtime for spawn_local")
             .block_on(async move {
                 let set = LocalSet::new();
                 send.send(set.spawn_local(fut()).into())
@@ -258,5 +272,6 @@ pub async fn spawn_local<
                 set.await
             })
     });
-    recv.await.unwrap()
+    recv.await
+        .expect("spawn_local thread terminated before sending task handle")
 }