Set the database constraints in the Meta class of the Idea model as follows:

# myproject/apps/ideas/models.py
from django.db import models
from django.utils.translation import gettext_lazy as _


class Idea(models.Model):
    author = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        verbose_name=_("Author"),
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
        related_name="authored_ideas",
    )
    title = models.CharField(
        _("Title"),
        max_length=200,
    )

    class Meta:
        verbose_name = _("Idea")
        verbose_name_plural = _("Ideas")
        constraints = [
            models.UniqueConstraint(
                fields=["title"],
                condition=~models.Q(author=None),
                name="unique_titles_for_each_author",
            ),
            models.CheckConstraint(
                check=models.Q(
                    title__iregex=r"^S.*S$"
                    # starts with non-whitespace,
                    # ends with non-whitespace,
                    # anything in the middle
                ),
                name="title_has_no_leading_and_trailing_whitespaces",
            )
        ]