fix: address #93 adversarial review findings

Backend: 401 documented on all mutating settings/resolution routes;
resolution delete distinguishes 404 (unknown) from 422 (not custom);
XMLTV enum bridging via exhaustive switch instead of int casts;
field-level Arg.Is assertions incl. non-null watermark/filler flow.

Frontend: resolution add/delete failures surfaced inline (were
silent); partial saves merge succeeded groups via allSettled; empty
numeric fields invalid + tunerCount min 1; HLS Direct select shows
out-of-list wire values; media-source rows show derived last-scan.

ErsatzTV.Tests 495; web suite 145.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/Controllers/Api/Requests/UpdateXmltvSettingsRequest.cs

@@ -11,7 +11,23 @@ public record UpdateXmltvSettingsRequest(int DaysToBuild, ApiXmltvTimeZone TimeZ
             new XmltvSettingsViewModel
             {
                 DaysToBuild = DaysToBuild,
-                TimeZone = (XmltvTimeZone)(int)TimeZone,
-                BlockBehavior = (XmltvBlockBehavior)(int)BlockBehavior
+                TimeZone = ToVmTimeZone(TimeZone),
+                BlockBehavior = ToVmBlockBehavior(BlockBehavior)
             });
+
+    private static XmltvTimeZone ToVmTimeZone(ApiXmltvTimeZone timeZone) =>
+        timeZone switch
+        {
+            ApiXmltvTimeZone.Local => XmltvTimeZone.Local,
+            ApiXmltvTimeZone.Utc => XmltvTimeZone.Utc,
+            _ => throw new ArgumentOutOfRangeException(nameof(timeZone), timeZone, null)
+        };
+
+    private static XmltvBlockBehavior ToVmBlockBehavior(ApiXmltvBlockBehavior blockBehavior) =>
+        blockBehavior switch
+        {
+            ApiXmltvBlockBehavior.SplitTimeEvenly => XmltvBlockBehavior.SplitTimeEvenly,
+            ApiXmltvBlockBehavior.UseActualTimes => XmltvBlockBehavior.UseActualTimes,
+            _ => throw new ArgumentOutOfRangeException(nameof(blockBehavior), blockBehavior, null)
+        };
 }