Define contract duration in number of blocks

This makes calculation with duration in Solidity simpler
and less prone to errors.

contracts/Contracts.sol

@@ -6,7 +6,7 @@ import "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
 contract Contracts {
 
   mapping(bytes32=>bool) private ids; // contract id, equal to hash of bid
-  mapping(bytes32=>uint) private durations; // contract duration in seconds
+  mapping(bytes32=>uint) private durations; // contract duration in blocks
   mapping(bytes32=>uint) private sizes; // storage size in bytes
   mapping(bytes32=>bytes32) private contentHashes; // hash of data to be stored
   mapping(bytes32=>uint) private prices; // price in coins

test/examples.js

@@ -1,7 +1,7 @@
 const { ethers } = require("hardhat")
 
 const exampleRequest = () => ({
-  duration: 31 * 24 * 60 * 60, // 31 days
+  duration: 200000, // 200,000 blocks ≈ 1 month
   size: 1 * 1024 * 1024 * 1024, // 1 Gigabyte
   contentHash: ethers.utils.sha256("0xdeadbeef"),
   proofPeriod: 8, // 8 blocks ≈ 2 minutes