build: migrate eslint to v10 flat config and upgrade typescript

Replace .eslintrc.json with eslint.config.js using the new flat config
format required by ESLint 10. Swap @typescript-eslint/eslint-plugin and
@typescript-eslint/parser for the unified typescript-eslint package.
Upgrade TypeScript to 5.9 and @types/node to 25. Apply automatic code
fixes: remove unused catch bindings and preserve error cause in auth
header parsing.

lib/forwarded.ts

@@ -20,12 +20,12 @@ const cidrs: [IPv4 | IPv6, number][] = [];
 for (const str of FORWARDED_HEADER.split(",").map((s) => s.trim())) {
   try {
     cidrs.push(parseCIDR(str));
-  } catch (err) {
+  } catch {
     // Not a valid CIDR format, try parsing as IP
     try {
       const ip = parse(str);
       cidrs.push([ip, ip.toByteArray().length * 8]);
-    } catch (err) {
+    } catch {
       // Not a valid IP either, ignore
     }
   }