Add board and list descriptions

[symonbaikov]

Summary

Adds optional descriptions for boards and lists so teams can document the intended workflow and purpose of each board/list directly in the board view.

Changes

• Added nullable description fields to boards and lists, including database migration, models, API validation, and Swagger docs.
• Added board description editing in Board Settings → General.
• Added list description editing from the list actions menu.
• Added compact plain-text description previews:
  • board descriptions display above the board action row
  • list descriptions display under the list title
• Adjusted board content spacing so the board action controls do not overlap cards when a board description is visible.
• Added backend contract tests and a small client utility test.

CLosed: #1485

Testing

• npm run server:lint
• PORT=1338 npm test --prefix server -- --exit
• npm run client:lint
• npm test --prefix client -- --runInBand
• npm run client:build
• Verified the migration on a clean temporary Postgres database with migrate up/down.
• Manually checked board and list descriptions in the running dev UI.