[Docs] Write docs/testnet-deployment.md — end-to-end guide from zero to funded escrow

[gregemax]

Summary

Issue #54 has been open since July 10 and is a Week 4 roadmap item. This issue specifies exactly what the guide must cover so a contributor can pick it up.

A new contributor should be able to follow docs/testnet-deployment.md alone and arrive at a funded escrow contract on Stellar testnet with backend calls succeeding.

Document outline

# Testnet Deployment Guide

## Prerequisites
- Rust toolchain (see rust-toolchain.toml)
- Stellar CLI (install instructions)
- Node.js 20+ / npm
- PostgreSQL 15+
- A Chromium browser + Freighter extension

## 1. Deploy the Soroban contract
  - Build WASM
  - Generate deployer keypair + fund via Friendbot
  - Deploy with stellar-cli
  - Record SOROBAN_CONTRACT_ID

## 2. Configure the backend
  - .env values: DATABASE_URL, JWT_SECRET, SOROBAN_CONTRACT_ID, STELLAR_NETWORK=testnet
  - Run migrations
  - Start backend

## 3. Verify EscrowService
  - Register two users (client + freelancer)
  - POST /contracts — create a contract
  - POST /contracts/:id/fund — decode the returned XDR in Stellar Lab
  - Expected: valid invokeHostFunction envelope targeting the contract

## 4. Sign and submit with Freighter
  - Install Freighter, switch to testnet
  - Import the test keypair
  - Fund escrow from the UI
  - Verify contract status → ACTIVE

## 5. Approve a milestone
  - Submit and approve a milestone
  - Verify the Payment record exists in the DB
  - Confirm the tx on stellar.expert

## Troubleshooting
  - Common Horizon RPC errors
  - XDR decode failures
  - Contract not found

Acceptance criteria

• A developer unfamiliar with Stellar can follow the guide without external research
• All commands are copy-pasteable and tested
• Closes issue [Docs] Write testnet deployment guide #54

Related

• Issue [Docs] Write testnet deployment guide #54 — original docs issue
• Issue [Smart Contract] Deploy escrow contract to Stellar testnet and wire SOROBAN_CONTRACT_ID to backend #99 — testnet deployment task (implementation side)
• docs/local-development.md — local setup (this guide extends it)