Review code quality and documentation

[kiku-jw]

Major refactoring:

• Split main.py (1349→307 lines) into modular route handlers
• Create app/dependencies.py with shared state and auth helpers
• Create app/routes/proxy.py for HTTP/LLM proxy endpoints
• Create app/routes/rapidapi.py for RapidAPI integration
• Update app/routes/health.py with all health check endpoints

Improved typing:

• Add Pydantic enums (HTTPMethod, MessageRole, CostPolicy)
• Add typed response models (TokenUsage, LLMResponseData)
• Add field validators with constraints (ge, le, min_length)
• Use Literal types for success/error flags

Code quality fixes:

• Replace print() with logger.warning() in analytics.py
• Remove unused imports
• Improve import organization

Documentation:

• Add docs/ARCHITECTURE.md with system overview
• Document request flow, design decisions, deployment

Testing:

• Add tests/test_routes_business.py for business routes
• Cover onboarding, analytics, health, and schema validation

Description

Brief description of changes

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

• Tests pass locally
• Added new tests
• Updated existing tests

Checklist

• Code follows style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated
• No new warnings generated