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/login_attempt.py

@@ -0,0 +1,21 @@
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, String, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class LoginAttempt(Base):
+    __tablename__ = "login_attempts"
+
+    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+    ip_address: Mapped[str] = mapped_column(String(64), nullable=False, index=True)
+    username: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    success: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+        index=True,
+    )