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>

src/analytics.ts

@@ -75,7 +75,7 @@ export function useScrollDepthTracking() {
 // --- Time on page ---
 export function useTimeOnPage() {
   const location = useLocation()
-  const enteredAt = useRef(Date.now())
+  const enteredAt = useRef(0)
   const path = useRef(location.pathname)
 
   useEffect(() => {