feat: Backend: Sandbox Mode for Test Wallets

[0xVida]

Description

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #136

Changes Made

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

Breaking Changes

Breaking Changes:

Migration Guide:

Screenshots/Demo

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes

[ogazboiz]

Excellent feature! The sandbox middleware architecture is clean: a single sandboxMiddleware that flags requests via header (X-Sandbox-Mode) or query param, injects a separate DB context, and returns clear X-Environment response headers and _sandbox metadata. This is exactly how production services isolate test traffic. The docs and env config are thorough. ✅ Merging.