Add RFC 5322 compliant Message-ID header to email notifications

[tkaufmann]

Summary

This PR implements automatic generation of RFC 5322 compliant Message-ID headers for all email notifications sent by diun.

Currently, diun email notifications do not include a Message-ID header, which can cause issues with:

• Email threading and tracking in mail clients
• Spam filtering systems
• RFC 5322 compliance

Changes

• New function generateMessageID() in internal/notif/mail/client.go:

  • Generates unique Message-IDs using nanosecond timestamps for high resolution
  • Includes 8 bytes of cryptographically secure random data (hex encoded)
  • Format: <timestamp.randomhex@domain>
  • Domain is taken from LocalName config if set, otherwise falls back to Host

• Integration in Send() method:

  • Message-ID is generated before sending each email
  • Header is set using mailMessage.SetHeader("Message-ID", messageID)

• Comprehensive unit tests in internal/notif/mail/client_test.go:

  • TestGenerateMessageID: Validates format with LocalName and Host fallback
  • TestGenerateMessageID_Uniqueness: Verifies uniqueness across 1000 iterations
  • TestGenerateMessageID_Format: Validates RFC 5322 format compliance

Benefits

✅ Improves email threading and tracking in mail clients
✅ Enhances spam filter compatibility
✅ Ensures RFC 5322 compliance for all outgoing notifications
✅ Each message gets a globally unique identifier

Testing

All tests pass successfully:

=== RUN   TestGenerateMessageID
=== RUN   TestGenerateMessageID/with_LocalName
=== RUN   TestGenerateMessageID/fallback_to_Host
--- PASS: TestGenerateMessageID (0.00s)
=== RUN   TestGenerateMessageID_Uniqueness
--- PASS: TestGenerateMessageID_Uniqueness (0.00s)
=== RUN   TestGenerateMessageID_Format
--- PASS: TestGenerateMessageID_Format (0.00s)
PASS
coverage: 16.4% of statements
ok  	github.com/crazy-max/diun/v4/internal/notif/mail

Tested with: docker buildx bake test

Implementation Details

The Message-ID generation follows RFC 5322 best practices:

• Uses highest-resolution clock (nanoseconds) for timestamp component
• Includes cryptographically secure randomness (64 bits) for uniqueness
• Properly formatted as <local-part@domain>
• Domain component ensures global uniqueness guarantee

Example generated Message-ID:

<1733837213123456789.a1b2c3d4e5f6a7b8@mail.example.com>

[crazy-max]

Can you squash your commits please?

Otherwise LGTM thanks