fix(playlists): re-adding a song to a playlist crashed with SQLiteConstraintException

(playlistId, songId) is the playlist_songs PK, so a second @Insert of the
same pair threw UNIQUE constraint failed and killed the app (reproduced
twice in the crash buffer on 10-03). Switch to OnConflictStrategy.IGNORE:
duplicate add is now a silent no-op. Verified on-device on 0.1.5 (7):
re-adding 'The Humbling River' to FAVORITES keeps the song list intact,
0 FATALs, no new crash-buffer entries.

app/build.gradle.kts

@@ -15,8 +15,8 @@ android {
         applicationId = "com.rada"
         minSdk = 26
         targetSdk = 35
-        versionCode = 6
+        versionCode = 7
-        versionName = "0.1.4"
+        versionName = "0.1.5"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }

app/src/main/java/com/rada/data/local/dao/PlaylistDao.kt

@@ -3,6 +3,7 @@ package com.rada.data.local.dao
 import androidx.room.Dao
 import androidx.room.Delete
 import androidx.room.Insert
+import androidx.room.OnConflictStrategy
 import androidx.room.Query
 import com.rada.data.local.entity.PlaylistEntity
 import com.rada.data.local.entity.PlaylistSongEntity
@@ -30,7 +31,9 @@ interface PlaylistDao {
     @Query("DELETE FROM playlists WHERE id = :playlistId")
     suspend fun deletePlaylistById(playlistId: Long)
 
-    @Insert
+    // IGNORE: (playlistId, songId) is the PK — re-adding a song that is
+    // already in the playlist must be a no-op, not a crash.
+    @Insert(onConflict = OnConflictStrategy.IGNORE)
     suspend fun addSongToPlaylist(playlistSong: PlaylistSongEntity)
 
     @Query("DELETE FROM playlist_songs WHERE playlistId = :playlistId AND songId = :songId")