Feature Request : Add Descriptive game mode

Add a new **Descriptive** game mode alongside the existing **Classic** mode.

The objective remains the same, but the gameplay differs from Classic.

## Gameplay

### Classic (Current)

- Existing gameplay remains unchanged.
- Uses the current grid generation and verification flow.

### Descriptive (New)

Each bingo cell contains a descriptive prompt instead of a letter or predefined value.

Examples:
- Person with no unread emails
- Wakes up without an alarm
- Has visited more than five countries
- Speaks three or more languages

When a participant finds someone matching a prompt, they can:

- Enter the participant's name.
- Optionally enter the participant verification code.
- Mark the square as completed.

Unlike Classic mode, participants complete their own grid independently after interacting with others.

## Verification

Verification code should be configurable by the organizer.

- Enabled → Participant must enter the verification code.
- Disabled → Only the participant's name is required.

## Backend Changes

- Add support for multiple game modes.
- Store the selected game mode for each event.
- Generate descriptive grids when Descriptive mode is selected.
- Support optional verification for Descriptive mode.
- Keep the existing Classic implementation unchanged.

Related

Frontend implementation will be tracked separately.

## Related

Frontend tracking: amfoss/ammingo-frontend/issues/33