fix: guard against self.unit being None in Job.__repr__

[juliosuas]

hit this on jobs that hadn't had a unit set yet:

TypeError: 'NoneType' object is not subscriptable

self.unit is initialized to None (there's even an Optional[str] type hint on it) but __repr__ slices it unconditionally:

self.unit[:-1] if self.interval == 1 else self.unit

so the moment you try to repr a freshly-constructed Job before the unit is set, it blows up. guarded both branches of the ternary with a None check and used "[unit not set]" as placeholder text.

fixes #646.

[juliosuas]

Quick note: the fix also handles the else-branch in __repr__ where the same self.unit[:-1] pattern appeared in the format string dict. Both paths now safely fall back to '[unit not set]' when self.unit is None. Normal jobs that have been assigned a unit are completely unaffected.