Fix plan form crash, badge contrast, target locking, schedule UX, sta…

[diegogodoy06]

…tus filter

• Fix AttributeError on POST /maintenance-plans/add/ on NetBox 4.6 / Django 6.0: NetBoxModelForm.clean() doesn't always return cleaned_data, so the prior super().clean().get(...) chain crashed. Read from self.cleaned_data instead.
• Fix unreadable Device / VM badges (grey bg + invisible white text) by switching to Bootstrap 5's text-bg-primary / text-bg-info utilities.
• Lock Device / VM fields client-side: picking one now disables the other. Server-side XOR validation stays as a safety net.
• Simplify the schedule UX on the plan form: relabel to Repeat every / Unit / Calendar anchor (optional), group with FieldSets, and add a worked example in the help text for the "1st of every month" use case.
• Add Status / Maintenance Type / Target filters on the Upcoming & Overdue Maintenance page. Status is computed in Python (multi-unit + anchored schedules can't be expressed as a single SQL predicate); the stats cards remain based on the unfiltered base set.
• Bump to 1.4.1, update CHANGELOG.