Feat/implement session bootstrap

[rabsqueen]

Summary

Improves the developer onboarding experience by documenting the required environment variables and local setup process for backend and payment integrations.
closes #660
closes #764
closes #762

Changes

• Added a comprehensive .env.example with documented placeholders for:
  • Database configuration
  • Authentication secrets
  • Redis
  • Stripe
  • USDC
  • Stellar Horizon and Soroban RPC endpoints
  • Feature flags
• Expanded the README.md with step-by-step local development instructions
• Updated docs/CONTRIBUTING.md with contributor setup, migration, testing, formatting, and linting workflows
• Included guidance for running both the TypeScript backend and Rust smart contract components

Benefits

• Simplifies onboarding for new contributors
• Reduces setup errors caused by missing environment variables
• Standardizes local development across backend and smart contract components
• Improves documentation without changing application behavior

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