Add super admin panel and update README

- Admin account bootstrapped from ADMIN_EMAIL/ADMIN_PASSWORD env vars on startup
- Admin panel: list users, view content, reset passwords, disable/delete accounts
- is_admin and is_disabled columns on users table
- Disabled accounts blocked at login
- README updated with admin setup instructions and panel docs

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

backend/auth.py

@@ -45,3 +45,9 @@ def get_current_user(token: str = Depends(oauth2_scheme), db: Session = Depends(
     if not user:
         raise exc
     return user
+
+
+def get_admin_user(current_user: User = Depends(get_current_user)) -> User:
+    if not current_user.is_admin:
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Admin access required")
+    return current_user