feat: [#279] add domain support for UDP trackers

[josecelano]

Summary

Closes #279

Add optional domain field to UDP tracker configuration to allow domain names in announce URLs instead of IP addresses. This feature mirrors the existing domain support in HTTP trackers.

Changes

Domain Layer (src/domain/tracker/config/udp.rs)

• Add domain: Option<DomainName> field to UdpTrackerConfig
• Include proper serde attributes for optional field serialization

Application Layer

• udp_tracker_section.rs: Add domain: Option<String> field with validation using DomainName::new()
• show/info/tracker.rs: Update to use domain for UDP URLs when available

Infrastructure Layer

• Update all test fixtures with domain: None for existing UDP tracker constructions

Schema & Config

• Regenerate JSON schema with new domain field description
• Update envs/manual-https-test.json with example UDP domain

Testing

• ✅ All unit tests pass (including 16 new tests for domain serialization)
• ✅ E2E infrastructure lifecycle tests pass
• ✅ E2E deployment workflow tests pass
• ✅ All linting checks pass (clippy, rustfmt, markdownlint, yamllint, shellcheck)
• ✅ Pre-commit verification complete

Example Usage

{
  "trackers": [
    {
      "name": "udp1",
      "udp": {
        "bind_port": 6969,
        "public_port": 6969,
        "domain": "udp1.tracker.example.com"
      }
    }
  ]
}

Before/After

Before: UDP URLs used IP addresses only

UDP announce URLs:
  Public IPv4: udp://10.123.219.117:6969
  Public IPv6: Not available

After: UDP URLs use domain when configured

UDP announce URLs:
  Public IPv4: udp://udp1.tracker.local:6969
  Public IPv6: Not available

[josecelano]

ACK a8bd087