Align error logging with admin dashboard field conventions

All warn/error log calls now use field names the dashboard's decorate()
function recognizes: `code` for the error code pill, `httpStatus` for the
status badge and severity bucket, `url` for the sidebar link, and
`repoId` for the repository link.

Key changes:
- Streamer errors surface code, httpStatus, url, and nested err in Raw tab
- Nested `{ err: serializeError(e) }` replaced with spread pattern so
  error fields (name, message, status) appear at the top level
- Raw Error objects in catch blocks now go through serializeError()
- Rate limit, token, and PR 404 warnings include code + httpStatus
- Dashboard stack walker traverses both `cause` and `err` chains
- Dashboard Raw tab renders repoId, filePath, upstream*, err, and cause
- trimRawArg recursively trims stacks in nested err/cause chains
- clampPayload strips heavy nested fields before falling back to
  truncated placeholder, preserving flat diagnostic fields

src/core/storage/S3.ts

@@ -132,7 +132,7 @@ export default class S3Storage extends StorageBase {
       try {
         res.status(500).json({ error: "file_not_found" });
       } catch (err) {
-        logger.error("send failed", { path, err: serializeError(err) });
+        logger.error("send failed", { ...serializeError(err), filePath: path });
       }
     }
   }