Enforce per-medium track cardinality in the upload service via MediumRules

Promote the Session/Mix single-track rule from a CMS-form convention to a
domain invariant: declare cardinality as data in MediumRules, enforce it in
UnifiedTrackService before the vault write (no orphan), return 409, and read
the same rule in the batch-form collapse.

DeepDrftModels/Enums/MediumRules.cs

@@ -0,0 +1,35 @@
+namespace DeepDrftModels.Enums;
+
+/// <summary>
+/// The allowed track-count range for a <see cref="ReleaseMedium"/>, expressed as an inclusive
+/// [Min, Max] band. <c>Max == int.MaxValue</c> denotes an unbounded (many-track) medium.
+/// </summary>
+public readonly record struct MediumCardinality(int Min, int Max)
+{
+    /// <summary>True when <paramref name="trackCount"/> falls within the inclusive band.</summary>
+    public bool Allows(int trackCount) => trackCount >= Min && trackCount <= Max;
+
+    /// <summary>True when the medium permits exactly one track (Max capped at 1).</summary>
+    public bool IsSingleTrack => Max == 1;
+}
+
+/// <summary>
+/// Single source of truth for per-medium structural rules. Today it declares track cardinality;
+/// the same table is read by the upload service (to reject over-limit track-adds) and the CMS
+/// batch forms (to decide whether to collapse the master list to one row), so the two cannot
+/// drift. A future medium declares its cardinality here — and only here — and every consumer
+/// honours it automatically. Pure declaration, no dependencies.
+/// </summary>
+public static class MediumRules
+{
+    private static readonly IReadOnlyDictionary<ReleaseMedium, MediumCardinality> Cardinalities =
+        new Dictionary<ReleaseMedium, MediumCardinality>
+        {
+            [ReleaseMedium.Cut] = new(1, int.MaxValue),
+            [ReleaseMedium.Session] = new(1, 1),
+            [ReleaseMedium.Mix] = new(1, 1),
+        };
+
+    /// <summary>The declared track-count band for <paramref name="medium"/>.</summary>
+    public static MediumCardinality CardinalityOf(ReleaseMedium medium) => Cardinalities[medium];
+}