feat(api): add schedule item duration estimates

refs #111

ErsatzTV.Application/ProgramSchedules/ProgramScheduleItemViewModel.cs

@@ -46,7 +46,7 @@ public abstract record ProgramScheduleItemViewModel(
 {
     /// <summary>
     ///     A rough estimate, in wall-clock time, of how long a single pass of this schedule item will play,
-    ///     derived from the aggregated runtimes (<c>MediaVersion.Duration</c>) of the referenced content.
+    ///     derived from the aggregated playout runtimes of the referenced content.
     ///     <para>
     ///         Semantics by <see cref="PlayoutMode" />:
     ///         <list type="bullet">
@@ -57,15 +57,12 @@ public abstract record ProgramScheduleItemViewModel(
     ///                 <see cref="MultipleMode.CollectionSize" />.
     ///             </item>
     ///             <item><b>Flood</b> — always <c>null</c>: a flood item fills the remaining time and is unbounded.</item>
-    ///             <item>
-    ///                 <b>Duration</b> — always <c>null</c>: its runtime is the explicit
-    ///                 <c>playoutDuration</c> setting already present on the item, so it is not re-derived here.
-    ///             </item>
+    ///             <item><b>Duration</b> — the explicit <c>playoutDuration</c> setting on the item.</item>
     ///         </list>
     ///     </para>
     ///     <para>
-    ///         <c>null</c> whenever a bounded estimate cannot be produced — an unbounded mode (Flood/Duration),
-    ///         a referenced collection with no items that have a known non-zero duration, a Multiple mode other
+    ///         <c>null</c> whenever a bounded estimate cannot be produced — an unbounded mode (Flood),
+    ///         a referenced collection with no items that have a known positive duration, a Multiple mode other
     ///         than Count/CollectionSize, or a collection type other than <see cref="CollectionType.Collection" />
     ///         (smart/multi/playlist/search/rerun/show/season/artist references are not aggregated in this pass).
     ///         Callers should treat a <c>null</c> as "unknown", never as zero.