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.

DeepDrftAPI/Startup.cs

@@ -43,6 +43,7 @@ namespace DeepDrftAPI
                 if (db is null) throw new Exception("Unable to initialize file database");
                 InitializeTrackVault(db).GetAwaiter().GetResult();
                 InitializeImageVault(db).GetAwaiter().GetResult();
+                InitializeMixWaveformsVault(db).GetAwaiter().GetResult();
                 return db;
             });
 
@@ -64,5 +65,14 @@ namespace DeepDrftAPI
                 await fileDatabase.CreateVaultAsync(VaultConstants.Images, MediaVaultType.Image);
             }
         }
+
+        // Ensure the mix-waveforms vault exists. Holds high-resolution waveform datums for DJ Mix releases.
+        private static async Task InitializeMixWaveformsVault(FileDatabase fileDatabase)
+        {
+            if (!fileDatabase.HasVault(VaultConstants.MixWaveforms))
+            {
+                await fileDatabase.CreateVaultAsync(VaultConstants.MixWaveforms, MediaVaultType.Media);
+            }
+        }
     }
 }