fix: production readiness — resolve build, lint, and code quality issues

- Fix Next.js build failure: remove duplicate route at (dashboard)/listings/[id]
  that conflicted with (public)/listings/[id] (same URL path in two route groups)
- Fix 772 ESLint errors: auto-fix import ordering (import-x/order), remove unused
  imports/variables, convert empty interfaces to type aliases, replace require()
  with ESM imports, fix consistent-type-imports violations
- Add CLAUDE.md for developer onboarding documentation
- All checks pass: 0 lint errors, typecheck clean, 230 tests passing, build success

Co-Authored-By: Paperclip <noreply@paperclip.ing>

scripts/seed-plans.ts

@@ -97,7 +97,7 @@ async function seedPlans() {
   console.log('Seeding subscription plans...\n');
 
   for (const plan of PLANS) {
-    const result = await prisma.plan.upsert({
+    const _result = await prisma.plan.upsert({
       where: { tier: plan.tier },
       update: {
         name: plan.name,