Add dual user classes: admin + tech users with PIN login

- Add tech user management (JSON-backed CRUD with PIN auth)
- Dual login: tabbed Tech Login (username+PIN) / Admin Login (NC credentials)
- Admin panel: tappable user list with detail modal (enable/disable, reset PIN, reset NC password, delete)
- Auto-provision Nextcloud accounts for tech users
- Admin guard: tech users redirected away from admin panel
- New data volume for persistent tech_users.json storage

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

docker-compose.yml

@@ -15,6 +15,7 @@ services:
       - TZ=${TZ:-UTC}
     volumes:
       - flask_sessions:/tmp/flask_session
+      - app_data:/app/data
     restart: unless-stopped
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8000/api/health"]
@@ -33,6 +34,8 @@ services:
 volumes:
   flask_sessions:
     driver: local
+  app_data:
+    driver: local
 
 networks:
   nextsnap-network: