Make MapWeight use exhaustive switch expression over ProvenanceKind

Replace if-chain with a switch expression that explicitly handles
Inferred and Extractive variants and throws ArgumentOutOfRangeException
for unrecognized enum values.

Author: Chris0Jeky

backend/src/Taskdeck.Application/Services/ProvenanceQueryService.cs

@@ -155,10 +155,11 @@ internal static string BuildValue(ProvenanceField field)
     /// </summary>
     internal static string MapWeight(ProvenanceKind kind, double confidence)
     {
-        if (kind == ProvenanceKind.Inferred)
-            return "inferred";
-
-        // Extractive: confidence determines primary vs contextual.
-        return confidence >= 0.7 ? "primary" : "contextual";
+        return kind switch
+        {
+            ProvenanceKind.Inferred => "inferred",
+            ProvenanceKind.Extractive => confidence >= 0.7 ? "primary" : "contextual",
+            _ => throw new ArgumentOutOfRangeException(nameof(kind), kind, "Unrecognized ProvenanceKind")
+        };
     }
 }