Docs: Onboarding — write comprehensive developer setup guide

[Hexstar-labs]

Description:
The README quick-start is minimal and skips many setup steps.

Tasks:

• Write docs/development-setup.md covering: prerequisites, env var setup, database setup, Redis setup, Stellar testnet account funding, contract deployment, running all services
• Add troubleshooting section for common errors
• Add a Makefile with targets: make setup, make dev, make test, make build
• Add a scripts/setup.sh that automates the full local setup

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