Separate probe target from display URL for monitors

[VrianCao]

Parent issue: #38

Summary

Support an optional display URL that is used in UI and notifications while keeping the probe target unchanged.

Scope

• Add an optional display_url field to monitor configuration
• Validate display_url as an HTTP or HTTPS URL
• Use display_url in admin UI and default notification templates when present
• Fall back to target when display_url is empty

Acceptance Criteria

• Monitoring behavior continues to use target only
• UI and webhook templates can access both target and display_url
• Existing monitors do not require data backfill
• Tests cover validation and fallback behavior