Security & hardening pass: scrypt password hashing, persisted admin sessions, upvote/analytics rate limits, dependency fixes, healthcheck, lint cleanup; v2.10.0

- Study passwords: salted scrypt; legacy SHA-256 hashes verified and upgraded on login
- Admin sessions persisted (hashed tokens) so deploys no longer log the admin out
- Upvotes: rate limit + one per client per question, approved questions only; rate limit on analytics play/event
- npm audit: 46 -> 4 vulnerabilities (tiptap bumped to 3.31.4)
- Untrack per-user data (study users/notes/progress/etc.) from git
- Dockerfile HEALTHCHECK; node --test suite for password hashing
- Fix email template cards remounting (lost input focus) on each keystroke
- Lint: 34 errors -> 0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/study-helpers.js

@@ -1,4 +1,4 @@
-import { createHash, createHmac, randomUUID } from 'node:crypto'
+import { createHash, createHmac, randomBytes, randomUUID, scryptSync, timingSafeEqual } from 'node:crypto'
 import path from 'node:path'
 import {
   parseCookies,
@@ -56,10 +56,43 @@ export function getStudySlugFromNoteId(sectionId) {
 
 // ── Password / token hashing ───────────────────────────────────────────────
 
-export function hashStudyPassword(password) {
+const LEGACY_HASH_RE = /^[0-9a-f]{64}$/
+
+function legacyHashStudyPassword(password) {
   return createHash('sha256').update(`study-user:${String(password)}`).digest('hex')
 }
 
+// New hashes are `scrypt$<salt hex>$<hash hex>`. Legacy accounts hold an
+// unsalted SHA-256 hex digest; verifyStudyPassword accepts both and
+// studyPasswordNeedsRehash lets login upgrade them transparently.
+export function hashStudyPassword(password) {
+  const salt = randomBytes(16)
+  const hash = scryptSync(String(password), salt, 64)
+  return `scrypt$${salt.toString('hex')}$${hash.toString('hex')}`
+}
+
+export function verifyStudyPassword(password, stored) {
+  if (typeof stored !== 'string' || !stored) return false
+  let expected
+  let actual
+  if (stored.startsWith('scrypt$')) {
+    const [, saltHex, hashHex] = stored.split('$')
+    if (!saltHex || !hashHex) return false
+    expected = Buffer.from(hashHex, 'hex')
+    actual = scryptSync(String(password), Buffer.from(saltHex, 'hex'), expected.length)
+  } else if (LEGACY_HASH_RE.test(stored)) {
+    expected = Buffer.from(stored, 'hex')
+    actual = Buffer.from(legacyHashStudyPassword(password), 'hex')
+  } else {
+    return false
+  }
+  return expected.length === actual.length && timingSafeEqual(expected, actual)
+}
+
+export function studyPasswordNeedsRehash(stored) {
+  return typeof stored === 'string' && !stored.startsWith('scrypt$')
+}
+
 export function hashEmailChangeToken(token) {
   return createHash('sha256').update(`study-email-change:${String(token)}`).digest('hex')
 }