Replace magic numbers with named constants and enums for clarity and maintainability

**Key Changes:**
- **Introduced type-safe enums** for better code clarity:
  - `RelayRole(Flag)` enum with HOP, STOP, CLIENT roles supporting bitwise combinations (e.g., `RelayRole.HOP | RelayRole.STOP`)
  - `ReservationStatus(Enum)` for reservation lifecycle management (ACTIVE, EXPIRED, REJECTED)
- **Replaced magic numbers with named constants** throughout the codebase, improving code maintainability and eliminating hardcoded timeout values (15s, 30s, 10s) with descriptive constant names
- **Added comprehensive timeout configuration system** with new `TimeoutConfig` dataclass supporting component-specific timeouts (discovery, protocol, DCUtR)
- **Enhanced configurability** of `RelayDiscovery`, `CircuitV2Protocol`, and `DCUtRProtocol` constructors with optional timeout parameters
- **Improved architecture consistency** with clean configuration flow across all circuit relay components
**Backward Compatibility:** All changes maintain full backward compatibility. Existing code continues to work unchanged while new timeout configuration options are available for users who need them.