feat: Enforces minimum length for registration reason

Adds a minimum length requirement of 30 characters to the
registration reason field to ensure more detailed responses.

src/synapse_registration/registration/forms.py

@@ -32,6 +32,7 @@ class RegistrationForm(forms.Form):
         )
     )
     registration_reason = forms.CharField(
+        min_length=30,
         widget=forms.Textarea(
             attrs={
                 "class": "textarea",