Authentication-aware link validation

[Mearman]

Problem

Currently, markmv reports authentication-protected URLs as broken links, even when they're functioning correctly. For example, Firebase Console URLs redirect to login pages, which markmv treats as validation failures.

Example

❌ [external] https://console.firebase.google.com/project/wagl-c3119/settings/general
   Reason: external-error

This URL is actually working correctly - it redirects to Google authentication as expected.

Proposed Solution

Add authentication-aware validation that can:

1. Distinguish between broken and auth-protected links

   • Detect common auth redirect patterns (Google, GitHub, Firebase Console, etc.)
   • Report auth-protected links differently from truly broken links

2. Support for authenticated validation

   • Optional API keys/tokens for validating protected documentation
   • Integration with common auth providers (Google, GitHub, etc.)

3. Configurable auth detection

   • Patterns for recognizing auth-protected domains
   • Custom redirect patterns that should be considered "valid"

Expected Output

✅ [external] https://console.firebase.google.com/project/wagl-c3119/settings/general
   Status: auth-required (redirects to login)
   
🔒 Authentication-protected links: 2
✅ Valid links: 160
❌ Broken links: 0

Use Cases

• Firebase Console project links
• GitHub private repository documentation
• Internal company documentation behind SSO
• API documentation requiring authentication

This would eliminate false positives and provide more accurate link validation for real-world documentation projects.