feat: SAC 0.7.2 security hardening (rate limit, JWT DB check, DOMPurify)

Path traversal fix in SPA fallback, admin-only host delete, login rate limit
with 3 attempts and Telegram alert, JWT validated against active users in DB,
and DOMPurify for agent report HTML.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.notification_channel import NotificationChannel
 from app.models.notification_cooldown import NotificationCooldown
 from app.models.notification_policy import NotificationPolicy
 from app.models.problem import Problem, ProblemEvent
+from app.models.login_attempt import LoginAttempt
 from app.models.event_severity_override import EventSeverityOverride
 from app.models.user import User
 
@@ -19,4 +20,5 @@ __all__ = [
     "ProblemEvent",
     "User",
     "EventSeverityOverride",
+    "LoginAttempt",
 ]