[Feature Request] Custom regex pattern for syntax error highlighting

[JohnLawes]

I'm working on an AI subtitle translation pipeline and would like to mark certain lines for manual review by adding tags like [CHECK] or [ERROR] in the subtitle text.

It would be very helpful if Subtitle Edit could support a custom regex pattern in Syntax Coloring settings, so that lines matching the pattern are highlighted as errors.

Use case:

• ASR confidence is low → mark with [CHECK]
• Timing adjustment needed → mark with [TIMING]
• Editor can quickly spot these lines and fix them

Proposed implementation:

• Add a text field in Options → Settings → Syntax Coloring for custom regex
• Lines matching the regex are highlighted with error color

This feature could also benefit other workflows:

• Team collaboration: reviewers mark lines with [TODO] or [QUESTION] for others
• QC pipelines: automated tools flag issues for human review
• Translation workflows: mark uncertain translations with [VERIFY]

Essentially, any scenario where subtitles pass through multiple stages or people could use custom markers for communication.