feat: global notification policy severity to channels (notif-22)

Singleton notification_policy table, NOTIFY_MIN_SEVERITY/CHANNELS env,
central dispatch gate, Settings policy UI, migration 007.

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

backend/app/models/notification_policy.py

@@ -0,0 +1,18 @@
+from sqlalchemy import Boolean, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+POLICY_ROW_ID = 1
+
+
+class NotificationPolicy(Base):
+    """Singleton: глобальное правило severity → каналы (F-NOT-02 урезанно)."""
+
+    __tablename__ = "notification_policy"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    min_severity: Mapped[str] = mapped_column(String(16), default="warning")
+    use_telegram: Mapped[bool] = mapped_column(Boolean, default=True)
+    use_webhook: Mapped[bool] = mapped_column(Boolean, default=False)
+    use_email: Mapped[bool] = mapped_column(Boolean, default=False)