Add subtitle field to column titles

[SelinaBuff]

### Discussed in https://github.com/inovex/scrumlr.io/discussions/4276

Description:**

Currently, our users can only enter a title for each column. To improve user experience, we should extend the functionality to include an additional field (below the title) for a short description/subtitle.

Acceptance Criteria:

• Users can successfully enter, save, and display a subtitle for each column.
• Changes to the subtitle can be made at any time (procedure: same as for the title).
• The layout remains clear and user-friendly.
• If necessary, a maximum character limit must be set, or the text should be truncated with (...) -> The full text will then be displayed using a tooltip.

Requirements:

Design Requirements:

• Integrate a field for the subtitle below the existing title field.
• Both fields (title and subtitle) must be clearly labeled to avoid confusion.
• Responsive design.
• Support for both dark and light modes.

Database Adjustments:

• Modify the database schema to enable storing subtitles.
• Ensure that existing data migration is performed without data loss.