fix(backend): cap vulnerability_report severity at Warning

A completed scan is a finding about the software inventory, not an
enrollment failure. Mapping the CVE risk onto Error/Critical made the
report read as a failed enrollment: the diagnosis page collected it as
failure evidence, MCP search lifted it on failure-intent queries, and the
timeline's Error filter filled up with clean sessions. critical/high now
map to Warning, everything else to Info; the CVE severity itself stays in
scan_summary and on the report panel's own scale.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions.Tests/VulnerabilityReportEventSequenceTests.cs

@@ -115,7 +115,7 @@ public class VulnerabilityReportEventSequenceTests
         Assert.Equal("vulnerability_report", evt.EventType);
         Assert.Equal(AutopilotMonitor.Shared.Constants.EventTypes.VulnerabilityReport, evt.EventType);
         Assert.Equal("VulnerabilityCorrelationService", evt.Source);
-        Assert.Equal(EventSeverity.Error, evt.Severity); // risk=high → Error
+        Assert.Equal(EventSeverity.Warning, evt.Severity); // risk=high → Warning (severity is capped)
         // Non-phase event: must stay Unknown so the UI timeline doesn't render a phantom phase boundary.
         Assert.Equal(EnrollmentPhase.Unknown, evt.Phase);
         Assert.Equal(NowUtc, evt.Timestamp);
@@ -127,6 +127,35 @@ public class VulnerabilityReportEventSequenceTests
         Assert.IsType<Dictionary<string, object>>(evt.Data["scan_summary"]);
     }
 
+    [Theory]
+    // The scan reports on the software inventory, not on the enrollment: a finding is never an
+    // enrollment failure, so severity is capped at Warning. Error/Critical would put the report
+    // into the diagnosis page's failure evidence and into MCP's failure-intent ranking.
+    [InlineData("critical", EventSeverity.Warning)]
+    [InlineData("high",     EventSeverity.Warning)]
+    [InlineData("medium",   EventSeverity.Info)]
+    [InlineData("low",      EventSeverity.Info)]
+    [InlineData("none",     EventSeverity.Info)]
+    [InlineData("HIGH",     EventSeverity.Warning)]  // case-insensitive
+    [InlineData("",         EventSeverity.Info)]
+    public void Severity_IsCappedAtWarning(string risk, EventSeverity expected)
+    {
+        var evt = Build(null, new List<EnrollmentEvent>(), Report(risk));
+
+        Assert.Equal(expected, evt.Severity);
+    }
+
+    [Fact]
+    public void Severity_IsInfo_WhenRiskIsMissing()
+    {
+        var report = new Dictionary<string, object>
+        {
+            ["scan_summary"] = new Dictionary<string, object>(),
+        };
+
+        Assert.Equal(EventSeverity.Info, Build(null, new List<EnrollmentEvent>(), report).Severity);
+    }
+
     [Fact]
     public void Data_Phase_CarriesTheLabel()
     {

src/Backend/AutopilotMonitor.Functions/Services/Vulnerability/VulnerabilityCorrelateHandler.cs

@@ -541,12 +541,26 @@ namespace AutopilotMonitor.Functions.Services.Vulnerability
             string.Equals(phase, "user_enrollment", StringComparison.Ordinal)
             || string.Equals(phase, "both", StringComparison.Ordinal);
 
+        /// <summary>
+        /// Maps the scan's overall CVE risk onto the event severity, capped at
+        /// <see cref="EventSeverity.Warning"/>.
+        /// <para>
+        /// <b>Why the cap:</b> <c>EventSeverity</c> on the timeline states how badly the
+        /// <i>enrollment</i> went, not how severe a CVE is. A completed scan is a finding about
+        /// the software inventory — nothing failed. Emitting Error/Critical made the report read
+        /// as an enrollment failure: the diagnosis page collects Error/Critical events as failure
+        /// evidence, MCP search lifts high-severity events on failure-intent queries, and the
+        /// timeline's Error filter — the one used to hunt for problems — filled up with clean
+        /// sessions. The CVE severity itself is not lost: it rides in <c>scan_summary</c>
+        /// (<c>overall_risk</c>, <c>critical_cves</c>, …) and the report panel renders it on its
+        /// own colour scale. Warning keeps the report visible without claiming a failure.
+        /// </para>
+        /// </summary>
         private static EventSeverity MapVulnerabilityRiskToSeverity(string? overallRisk) =>
             (overallRisk ?? string.Empty).ToLowerInvariant() switch
             {
-                "critical" => EventSeverity.Critical,
-                "high"     => EventSeverity.Error,
-                "medium"   => EventSeverity.Warning,
+                "critical" => EventSeverity.Warning,
+                "high"     => EventSeverity.Warning,
                 _          => EventSeverity.Info,
             };
     }