feat: add faculty assignments with grading and scheduled grade publishing

[ishad05]

What this adds

• Faculty can now create assignments right from the portal, with a live preview of how it will look to students
• Faculty get an assignments list and a detail page to review who submitted what
• Faculty can grade submissions and leave remarks

Grading and publishing

• Grades can be saved as a draft, published now, or scheduled to publish later
• Scheduled grades go live automatically when their time comes
• The student only sees a grade once it is published

Tested

• Ran the whole flow end to end: faculty creates an assignment, student submits, faculty grades it, student sees the published grade
• Checked all three publish options and the scheduled auto publish