Add per-user AI usage quota tracking

[knoxiboy]

Description

Track AI usage per user to control costs and support future pricing or institution-level limits.

Problem

Without AI usage tracking, project admins cannot understand or control Groq API cost drivers. The /api/ask-ai and /api/doubts/check-similarity routes make Groq calls with no per-user accounting.

Proposed Solution

1. Add an ai_usage table with: userEmail, route, model, tokensUsed (from Groq response), timestamp. 2. Log usage after each successful Groq call. 3. Add a daily/monthly quota check before making Groq calls. 4. Expose usage stats to admin dashboard.

Files Affected

• src/configs/schema.ts -- add aiUsageTable
• [NEW] drizzle/0009_ai_usage.sql -- migration
• src/app/api/ask-ai/route.ts -- log usage after line 466
• src/app/api/doubts/check-similarity/route.ts -- log usage after line 95
• [NEW] src/lib/ai/usage-tracker.ts -- track and check quota

Implementation Checklist

• ai_usage table exists with user, route, model, and token fields
• AI usage is recorded after each successful Groq call
• Daily/monthly quota checks can be performed before calling Groq
• Quota exceeded returns 429 without calling the model provider
• Admin or debug queries can inspect per-user usage

Additional Context

This is a GSSoC'26 contributor issue created from the DoubtDesk market-readiness roadmap.

Field	Value
Level	intermediate
Area	backend
Stack	Next.js, TypeScript, Drizzle, Neon Postgres, Clerk, Groq, Inngest

[Aditya8369]

@knoxiboy please assign me