feat(ai): Implement AI Essay & Assignment Grader

[rach-kanc]

Description

This Pull Request introduces the AI Essay & Assignment Grader feature, providing students with immediate, structured, and actionable feedback on their writing.

Motivation: Students need a quick way to self-evaluate drafts and assignments before final submission to catch structural flaws and improve their writing quality.
Context:

• Created a new essay_evaluations table in Supabase with RLS to securely store past submissions and feedback.
• Implemented a structured prompt in lib/ai.ts using Gemini to return exact JSON matching an EssayFeedback interface (estimated grade, pros, cons, and actionable suggestions with direct quotes).
• Added a new backend API endpoint that validates the request, consumes user credits, calls the AI, and saves the data.
• Built a dashboard UI to submit essays (with optional grading rubrics) and a detailed results page to view the feedback.

Related Issue: Closes #79

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Note: This project does not currently have an automated test suite. The following manual verification was performed:

• Backend/AI: Verified that the AI consistently returns valid JSON matching the strict required schema. Checked that consumeCredit accurately deducts 1 credit per evaluation.
• Database: Confirmed that RLS policies prevent users from viewing evaluations belonging to other accounts.
• UI Testing: Verified the form handles loading states properly and the results page renders Strengths, Weaknesses, and Suggestions clearly.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings/lint errors
• I have checked that my branch is up-to-date with main
• All tests pass locally with my changes (N/A - no local tests exist)

##Tags
ssoc26, hard

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[rach-kanc]

@MistryVishwa