fix: use Prisma.DbNull to filter non-null Json? field in analysisOnly

Prisma 5 rejects JS null as a where-clause value on Json? fields.
Use Prisma.DbNull (SQL NULL sentinel) so the NOT filter compiles correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/experiments.js

@@ -1,5 +1,6 @@
 const router = require('express').Router();
 const { body } = require('express-validator');
+const { Prisma } = require('@prisma/client');
 const prisma = require('../lib/prisma');
 const cache = require('../lib/cache');
 const auditMiddleware = require('../middleware/auditMiddleware');
@@ -373,7 +374,7 @@ router.get('/:id/daily-statuses', async (req, res, next) => {
       where.date = { gte: d, lt: next };
     }
     if (analysisOnly) {
-      where.NOT = { analysis_summary: null };
+      where.NOT = { analysis_summary: Prisma.DbNull };
     }
 
     const statuses = await prisma.dailyStatus.findMany({