fix: prevent information disclosure in error handling

[akshayurankar48]

Summary

• Replace DateTime exception echo with error_log() to prevent leaking PHP internals (file paths, timezone info) to unauthenticated frontend visitors
• Return $content unchanged on error instead of bare return
• Sanitize field_id parameter in oEmbed AJAX response with sanitize_text_field(wp_unslash()) before output

Test plan

• Verify video schema still renders correctly on frontend
• Verify invalid video dates are handled gracefully (no visible error)
• Check error log for DateTime errors instead of frontend output
• Verify oEmbed embed/remove still works in post editor

Fixes #222