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>

Dockerfile

@@ -35,4 +35,6 @@ COPY entrypoint.sh ./entrypoint.sh
 RUN chmod +x ./entrypoint.sh
 
 EXPOSE 4173
+HEALTHCHECK --interval=30s --timeout=5s --start-period=30s --retries=3 \
+  CMD node -e "fetch('http://127.0.0.1:'+(process.env.PORT||4173)+'/api/admin-auth/status').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
 ENTRYPOINT ["./entrypoint.sh"]