Add FastAPI integration with dependencies

[genro]

Context

Users need easy way to protect FastAPI endpoints with token authentication. Standard FastAPI approach is dependency injection.

Goal

Create FastAPI dependencies for authentication and scope-based authorization.

Implementation

• Create genro_auth/fastapi.py module
• Implement create_auth_dependency(token_manager) function
• Implement create_scope_dependency(required_scopes) function
• Use HTTPBearer for token extraction from Authorization header
• Return user data dict from dependency
• Raise HTTPException(401) for invalid/missing tokens
• Raise HTTPException(403) for insufficient scopes

Acceptance Criteria

• require_auth = create_auth_dependency(manager) works
• @app.get('/protected') with Depends(require_auth) protects endpoint
• Invalid token returns 401 with clear message
• Missing scopes returns 403 with clear message
• User data accessible in route: user['user_id'], user['scopes']

Dependencies

• Requires: Add declarative configuration API #2 (needs configured TokenManager)
• Blocks: Add FastAPI integration tests #5 (integration tests need FastAPI integration)

Notes

• Use standard FastAPI Depends() pattern
• Extract token from Authorization: Bearer <token> header
• Scope check: exact match (no wildcards in v0.1.0)
• Return user dict: {'user_id': str, 'scopes': list, 'type': str, 'expires_at': datetime}