feat(remote): add DatabaseService.fetchTracksByIds for efficient ID-based lookups

Music/Services/DatabaseService.swift

@@ -201,6 +201,20 @@ nonisolated final class DatabaseService: Sendable {
         }
     }
 
+    func fetchTracksByIds(_ ids: [Int64]) throws -> [Track] {
+        guard !ids.isEmpty else { return [] }
+        let tracks = try dbPool.read { db in
+            let placeholders = databaseQuestionMarks(count: ids.count)
+            return try Track.fetchAll(
+                db,
+                sql: "SELECT * FROM tracks WHERE id IN (\(placeholders))",
+                arguments: StatementArguments(ids)
+            )
+        }
+        let trackMap = Dictionary(uniqueKeysWithValues: tracks.compactMap { t in t.id.map { ($0, t) } })
+        return ids.compactMap { trackMap[$0] }
+    }
+
     func fetchRecentlyAdded(limit: Int) throws -> [Track] {
         try dbPool.read { db in
             try Track.fetchAll(

MusicTests/DatabaseServiceTests.swift

@@ -264,6 +264,26 @@ struct DatabaseServiceTests {
         #expect(total == 0)
     }
 
+    // Inserts 5 tracks, fetches 3 by ID, verifies only those 3 are returned
+    // in the order of the requested IDs.
+    @Test func fetchTracksByIds() throws {
+        let db = try DatabaseService(inMemory: true)
+        var tracks = (0..<5).map { i in
+            Track.fixture(fileURL: "/track\(i).mp3", title: "Track \(i)")
+        }
+        for i in tracks.indices {
+            try db.insert(&tracks[i])
+        }
+
+        let ids: [Int64] = [tracks[2].id!, tracks[0].id!, tracks[4].id!]
+        let result = try db.fetchTracksByIds(ids)
+
+        #expect(result.count == 3)
+        #expect(result[0].id == tracks[2].id)
+        #expect(result[1].id == tracks[0].id)
+        #expect(result[2].id == tracks[4].id)
+    }
+
     // Inserts tracks in different months and verifies fetchMonthlyAdditions returns
     // the correct per-month counts covering the requested range including empty months.
     // Uses a UTC calendar to match the implementation, which uses UTC month boundaries