docs: improve dev onboarding experience

[forge0x]

What

Improves the developer onboarding experience for new contributors setting up the Flash backend locally.

Changes

1. DEV.md — Complete rewrite

• Quick Start section (3 commands to running server)
• Prerequisites table with exact versions and install links
• Step-by-step manual setup with copy-pasteable commands
• Architecture diagram showing all services and ports
• Troubleshooting section covering the most common issues:
  • UnauthorizedError (use oathkeeper proxy, not direct port)
  • Node engine incompatibility
  • Docker variable warnings
  • SendGrid API key warning
• Useful commands reference table

2. dev/setup.sh — New interactive setup script

• Validates Node.js version (auto-switches via nvm if available)
• Checks yarn, Docker, direnv
• Prompts for Ibex sandbox credentials and writes .env.local
• Generates dev-overrides.yaml automatically
• Installs dependencies
• Starts Docker containers
• Prints success summary with next steps

3. package.json — Fix engine spec

• Changed "node": "20" → "node": ">=20.18.1 <21"
• The cheerio@1.1.2 dependency requires >=20.18.1, so 20.10.0 fails on yarn install
• The bare "20" spec was too loose — it matched Node 20.10.0 which then broke on transitive deps

Issues Found During Setup

1. Node version: "node": "20" in package.json matches any 20.x, but cheerio needs >=20.18.1 → confusing error for devs using 20.10.0
2. No quick start: DEV.md assumed familiarity with direnv, config structure, and credential flow
3. No troubleshooting: Common errors (auth error on direct port, SendGrid warning) had no documentation
4. Manual credential setup: Required reading multiple files to understand where credentials go

Testing

• Cloned fresh, ran ./dev/setup.sh, then make start — server started and responded on all endpoints
• Verified GraphQL queries work through oathkeeper proxy at localhost:4002