Enforce minimum ten classes per module

[cdsap]

Summary:

• document the new minimum classes-per-module constraint and adjust descriptions/examples in the README
• clamp web input to between 10 and 15, honour the new default in backend/CLI/ProjectGenerator, and add validation/tests around the ClassesPerModule model
• ensure random generation still respects the minimum when upper bounds equal the minimum

Testing:

• Not run (not requested)