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>

src/fabledassistant/config.py

@@ -25,4 +25,5 @@ class Config:
     OLLAMA_URL: str = os.environ.get("OLLAMA_URL", "http://localhost:11434")
     OLLAMA_MODEL: str = os.environ.get("OLLAMA_MODEL", "llama3.1")
     SECRET_KEY: str = _read_secret("SECRET_KEY", "SECRET_KEY_FILE", "dev-secret-change-me")
+    SECURE_COOKIES: bool = os.environ.get("SECURE_COOKIES", "").lower() in ("1", "true", "yes")
     LOG_LEVEL: str = os.environ.get("LOG_LEVEL", "INFO")