fix: add missing errors utility and fix seed timestamp casting

- Add apps/api/src/utils/errors.ts with AppError class
- Fix seed.ts timestamp casting for calendario_fiscal

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/prisma/seed.ts

@@ -216,7 +216,7 @@ async function main() {
       await prisma.$executeRawUnsafe(`
         INSERT INTO "${schemaName}"."calendario_fiscal"
         (titulo, descripcion, tipo, fecha_limite, recurrencia, completado)
-        VALUES ($1, $2, $3, $4, 'mensual', $5)
+        VALUES ($1, $2, $3, $4::timestamp, 'mensual', $5)
         ON CONFLICT DO NOTHING
       `, evento.titulo, `${evento.titulo} - ${mes}/${año}`, evento.tipo, fechaLimite.toISOString(), completado);
     }