fix: stage audio uploads on data disk instead of /tmp

Relocate both the framework multipart buffer (via ASPNETCORE_TEMP) and the controller staging file to a configurable data-disk directory, so large WAV/FLAC/MP3 uploads no longer fail on the host's small tmpfs.

DeepDrftAPI/Models/UploadSettings.cs

@@ -0,0 +1,13 @@
+namespace DeepDrftAPI.Models
+{
+    /// <summary>
+    /// Non-secret upload tunables. <see cref="StagingPath"/> is the directory used to stage the raw
+    /// audio body during upload/replace-audio. It must live on the data disk, never the system temp
+    /// mount (on the Linux host <c>/tmp</c> is a small RAM-backed tmpfs that cannot hold a multi-hundred-MB
+    /// WAV). When null/empty it defaults to a "staging" subdirectory under the FileDatabase vault path.
+    /// </summary>
+    public class UploadSettings
+    {
+        public string? StagingPath { get; set; }
+    }
+}

DeepDrftAPI/Models/UploadStagingDirectory.cs

@@ -0,0 +1,10 @@
+namespace DeepDrftAPI.Models
+{
+    /// <summary>
+    /// The resolved, on-disk staging directory for upload/replace-audio bodies. Resolved once at
+    /// startup from <see cref="UploadSettings"/> (or the vault path default) and guaranteed to exist.
+    /// Injected into <c>TrackController</c> so the upload path never stages on the system temp mount.
+    /// A typed wrapper rather than a bare string so DI resolves it unambiguously.
+    /// </summary>
+    public sealed record UploadStagingDirectory(string Path);
+}