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/tests/test_settings_api.py

@@ -22,6 +22,8 @@ def test_get_notification_settings_masks_secrets(jwt_headers, client, monkeypatc
     assert body["telegram"]["min_severity"] == "warning"
     assert body["telegram"]["source"] == "env"
     assert body["telegram"]["bot_token_hint"].endswith("ghij")
+    assert "policy" in body
+    assert body["policy"]["min_severity"] == "warning"
     assert "webhook" in body
     assert body["webhook"]["enabled"] is False
     assert "email" in body
@@ -38,7 +40,6 @@ def test_put_email_settings_persists_db(jwt_headers, client, db_session, monkeyp
         headers=jwt_headers,
         json={
             "enabled": True,
-            "min_severity": "warning",
             "smtp_host": "smtp.example.com",
             "smtp_port": 587,
             "smtp_user": "monitor",
@@ -95,7 +96,6 @@ def test_put_webhook_settings_persists_db(jwt_headers, client, db_session, monke
         headers=jwt_headers,
         json={
             "enabled": True,
-            "min_severity": "warning",
             "url": "https://example.com/hooks/sac",
             "secret_header": "X-SAC-Token",
             "secret": "supersecret",
@@ -145,7 +145,6 @@ def test_put_telegram_settings_persists_db(jwt_headers, client, db_session, monk
         headers=jwt_headers,
         json={
             "enabled": True,
-            "min_severity": "warning",
             "bot_token": "1234567890:TESTTOKEN",
             "chat_id": "999001",
         },
@@ -154,7 +153,6 @@ def test_put_telegram_settings_persists_db(jwt_headers, client, db_session, monk
     body = response.json()
     assert body["source"] == "db"
     assert body["enabled"] is True
-    assert body["min_severity"] == "warning"
 
     row = db_session.get(NotificationChannel, "telegram")
     assert row is not None
@@ -163,7 +161,6 @@ def test_put_telegram_settings_persists_db(jwt_headers, client, db_session, monk
 
     cfg = get_effective_telegram_config(db_session)
     assert cfg.source == "db"
-    assert cfg.min_severity == "warning"
 
 
 def test_post_telegram_test_success(jwt_headers, client, db_session, monkeypatch):