Add examples and update documentation

[genro]

Context

Users need practical examples showing how to use genro-auth in real applications. Examples are better than lengthy docs.

Goal

Create 3 working examples and update README with provider pattern.

Implementation

• Create examples/basic_usage.py
  • TokenManager setup
  • Generate token
  • Validate token
  • Refresh token
  • Revoke token
• Create examples/fastapi_app.py
  • Complete FastAPI application
  • Protected endpoints
  • Login endpoint (token generation)
  • Refresh endpoint
  • Scope-based authorization
• Create examples/custom_storage.py
  • Implement custom TokenStorage
  • Example with SQLite
  • Show how to extend
• Update README.md
  • Add provider pattern explanation
  • Add configuration examples
  • Update quick start
  • Add links to examples
• Create docs/ARCHITECTURE.md (optional)
  • Explain provider pattern
  • Explain storage abstraction

Acceptance Criteria

• All examples run without errors
• Examples are self-contained (can copy-paste)
• README reflects new architecture
• Code in examples is production-quality (not toy code)

Dependencies

• Requires: Refactor to provider pattern for multi-token-type support #1, Add declarative configuration API #2, Add FastAPI integration with dependencies #3 (all features implemented)
• Can start after Add comprehensive unit tests #4 (tests don't block examples)

Notes

• Examples should be commented
• Show best practices (error handling, logging)
• Keep examples simple but realistic
• Add comments explaining key concepts