fix(mcp): get_monitor says how often a monitor on the global interval runs (#5433)

check_interval_s was `m.check_interval_seconds or None`, so a monitor
following monitors.poll_interval_seconds read as null and a reader
couldn't tell how often it ran. Monitor results now carry the effective
interval and `interval_from: "monitor" | "global"`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_mcp_monitoring_writes.py

@@ -76,6 +76,9 @@ def test_monitor_rule_ack_and_maintenance_writes(app):
         assert made["target"] == "https://nas.lan" and made["host_name"] == host_name
         assert made["config"]["headers"] == {"Authorization": "<redacted>"}
         assert made["config"]["verify_ssl"] is True
+        # No interval of its own: it says which it follows, and how often (#5433).
+        assert (made["check_interval_s"], made["interval_from"]) == (
+            int(app.config.get("MONITORS_POLL_INTERVAL", 60)), "global")
 
         # ── rules ──────────────────────────────────────────────────────────
         opts = await a.get_alert_rule_options("http")
@@ -100,7 +103,9 @@ def test_monitor_rule_ack_and_maintenance_writes(app):
         assert renamed["carried"]["from"] == mon_name and renamed["carried"]["rules"] == 1
         async with app.db_sessionmaker() as s:
             assert (await s.get(AlertRule, rule["rule_id"])).resource_name == f"{mon_name}-2"
-        assert "carried" not in await m.update_monitor(f"{mon_name}-2", check_interval_s=30)
+        own = await m.update_monitor(f"{mon_name}-2", check_interval_s=30)
+        assert "carried" not in own
+        assert (own["check_interval_s"], own["interval_from"]) == (30, "monitor")
         # Another http monitor may not take the same name: they would share one history.
         with pytest.raises(ValueError, match="already named"):
             await m.create_monitor(f"{mon_name}-2", "http", "other.lan")