docs: add comprehensive CLAUDE.md guide for future development

Create detailed documentation for Claude Code to understand the project
architecture and development workflows.

Contents:
- Project overview and technology stack
- Common development commands (setup, testing, docker, migrations)
- Architecture deep dive (app, db, models, tests structure)
- Configuration management and environment variables
- Security implementation guide with CI/CD troubleshooting
- Testing strategy and patterns
- Code quality standards (formatting, linting, type hints)
- Common development tasks (adding endpoints, tables, features)
- Debugging guide and common issues
- Production deployment checklist

Security Implementation Guide includes:
- Exact solutions for Bandit, Black, Ruff, MyPy issues
- Slowapi parameter naming requirements
- Type annotation patterns for third-party libraries
- 15-commit sequence from successful Phase 2.2 implementation
- Common pitfalls and how to avoid them
- Test coverage requirements

This guide enables future Claude instances to be productive immediately
and avoid the CI/CD failures encountered during security implementation.

Author: Sakeeb91