Add registration control, admin user management, and security hardening

- Registration auto-closes after first user; admin can toggle from /admin/users
- Admin user management view with user list and delete
- Password confirmation on registration form
- Password change in Settings (PUT /api/auth/password)
- Session cookie hardening: HttpOnly, SameSite=Lax, optional Secure flag
- Startup warning when SECRET_KEY is default
- Production deployment docs: reverse proxy, rate limiting, CSP headers
- Fix assist prompt to preserve markdown headings in target sections
- Simplify prod compose networking

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

docker-compose.prod.yml

@@ -10,7 +10,6 @@ services:
       OLLAMA_MODEL: "${OLLAMA_MODEL:-llama3.1}"
       LOG_LEVEL: "${LOG_LEVEL:-INFO}"
     networks:
-      - fabledassistant_frontend
       - fabledassistant_backend
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:5000/api/health')"]
@@ -80,8 +79,5 @@ volumes:
   ollama_models:
 
 networks:
-  fabledassistant_frontend:
-    driver: overlay
   fabledassistant_backend:
     driver: overlay
-    internal: true