Phase 9 Wave 2: api/release endpoint family — medium-aware reads + metadata writes

Adds ReleaseRepository/ReleaseManager (IReleaseService) for paged medium-filtered
release reads and Session/Mix satellite writes, UnifiedReleaseService orchestrating
vault+SQL, and ReleaseController (5 endpoints). Refactors WaveformProfileService for
configurable bucketCount/vaultName (backward-compatible) and adds the mix-waveforms vault.
Promotes brittle error-string literals to named constants (MixHasNoTrackMessage,
MixTrackNoAudioMessage) on UnifiedReleaseService.

DeepDrftContent/Constants/VaultConstants.cs

@@ -20,4 +20,10 @@ public static class VaultConstants
     /// from <c>TrackEntity.ImagePath</c>.
     /// </summary>
     public const string Images = "images";
+
+    /// <summary>
+    /// Vault name for Mix high-resolution waveform datums, keyed by the mix track's EntryKey.
+    /// Distinct from WaveformProfiles (player-bar low-res); same pipeline at higher resolution.
+    /// </summary>
+    public const string MixWaveforms = "mix-waveforms";
 }