Bug fixes (real breakage):

Newsletter nudge — was calling /api/study-account/profile (wrong endpoint, ignored subscription). Now correctly calls /api/study-account/preferences with PATCH.
Security:

Email regex — replaced the permissive [^\s@]+@[^\s@]+ pattern with a proper RFC-compliant regex in contact.js and downloads.js
Avatar magic bytes — server now checks actual PNG/JPEG/GIF/WEBP header bytes, not just the data URL prefix
Certificate rate limit — public /api/public/certificate/:token now has a 30 req/15min limiter
Session absolute TTL — admin sessions now have a 30-day hard cap; a stolen token can no longer be kept alive indefinitely by passive reads
Account lockout — 5 failed logins locks a study account for 1 hour
CSP headers — Content-Security-Policy, X-Frame-Options, X-Content-Type-Options, and Referrer-Policy headers added globally
Data integrity:

Cascade delete — deleting a study account now also removes their certificates, community posts, comments, and progress file
UX / reliability:

Escape key on modals — all 3 modal groups (study index, notes, account) now close on Escape
Display name min-length — empty spaces-only names rejected; if provided, must be ≥2 chars
Note save rate limit — 30 saves/minute per user max
Analytics fetch timeout — 5s AbortController so a hanging server doesn't block the browser indefinitely
Email validation on signup — frontend catches bad email formats before hitting the server
Cleanup:

Deduplicated download forms — StudyDownloadForm and ResourceDownloadForm now share a single DownloadForm base; both are now thin wrappers

server/routes/study-certificate.js

@@ -1,9 +1,18 @@
 import { randomUUID } from 'node:crypto'
+import rateLimit from 'express-rate-limit'
 import { requireAdminAuth } from '../auth.js'
 import { requireStudyAuth, normalizeStudySlug, isStudyUserEnrolled, getStudyTitleBySlug } from '../study-helpers.js'
 import { state } from '../state.js'
 import { loadUserProgress, queueStudyCertificatesWrite } from '../data.js'
 
+const publicCertRateLimiter = rateLimit({
+  windowMs: 15 * 60 * 1000,
+  max: 30,
+  standardHeaders: true,
+  legacyHeaders: false,
+  message: { message: 'Too many certificate lookups. Please wait a moment.' },
+})
+
 /**
  * Returns the list of required section IDs for a study (only released sections).
  */
@@ -92,7 +101,7 @@ export function register(app) {
   })
 
   // GET — public certificate page data (no auth, by token)
-  app.get('/api/public/certificate/:token', (req, res) => {
+  app.get('/api/public/certificate/:token', publicCertRateLimiter, (req, res) => {
     const { token } = req.params
     if (!token || typeof token !== 'string' || token.length > 100) {
       res.status(400).json({ message: 'Invalid token.' }); return