feat: add LibraryViewModel with reactive queries, debounced search, and column sorting

Music/ViewModels/LibraryViewModel.swift

@@ -0,0 +1,63 @@
+import Foundation
+import Observation
+import GRDB
+
+@Observable
+final class LibraryViewModel {
+    var tracks: [Track] = []
+    var searchText = ""
+    var sortColumn = "title"
+    var sortAscending = true
+    var trackCount = 0
+
+    private let db: DatabaseService
+    private var cancellable: AnyDatabaseCancellable?
+    private var searchTask: Task<Void, Never>?
+
+    init(db: DatabaseService) {
+        self.db = db
+        updateQuery()
+    }
+
+    func search(_ text: String) {
+        searchText = text
+        searchTask?.cancel()
+        searchTask = Task { @MainActor [weak self] in
+            try? await Task.sleep(for: .milliseconds(150))
+            guard !Task.isCancelled else { return }
+            self?.updateQuery()
+        }
+    }
+
+    func sort(by column: String) {
+        if sortColumn == column {
+            sortAscending.toggle()
+        } else {
+            sortColumn = column
+            sortAscending = true
+        }
+        updateQuery()
+    }
+
+    private func updateQuery() {
+        cancellable?.cancel()
+        let search = searchText
+        let col = sortColumn
+        let asc = sortAscending
+
+        let observation = ValueObservation.tracking { [db] dbAccess in
+            try db.fetchTracks(db: dbAccess, search: search, sortColumn: col, ascending: asc)
+        }
+
+        cancellable = observation.start(
+            in: db.dbPool,
+            onError: { error in
+                print("Library observation error: \(error)")
+            },
+            onChange: { [weak self] tracks in
+                self?.tracks = tracks
+                self?.trackCount = tracks.count
+            }
+        )
+    }
+}