Support additional sports (soccer, ...)

Follow-up to the MVP (#1), which supports only Ultimate Frisbee.

## Feature

Add field specs for other sports, e.g.:

- **Soccer**: full-size and small-sided (5v5/7v7/9v9) pitches — corners, penalty areas, center circle reference points.
- Others over time: flag football, lacrosse, rugby, cricket boundary, etc.

## Notes

- Requires generalizing the marker model beyond "8 corner cones": arbitrary sets of named marker points (and possibly line/arc guidance for painting lines).
- Pairs with the custom-dimensions work in the additional-field-specs issue.

Start by reviewing the MVP issue (#1) and the additional-field-specs work, then locate the field-spec and marker model. Define how named marker points represent soccer layouts and how this relates to custom dimensions; done should include full-size and small-sided soccer specifications without limiting fields to eight corner cones.

Written by the indexing model from the issue text.