step 2: Add UUID columns with backfill and dual-write support

- Add migration 000007: Add UUID columns to soundtrack and song with backfill
- Update InsertSoundtrack and InsertSoundtrackWithExistingId to accept UUID
- Update AddSong to accept UUID
- Add dual-write: Go code now generates UUIDs for new records
- Add uuid and pgtype imports

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/db/repository/models.go

@@ -10,14 +10,6 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-type IDMigrationStatus struct {
-	TableName    string     `json:"table_name"`
-	TotalRows    int32      `json:"total_rows"`
-	MigratedRows int32      `json:"migrated_rows"`
-	Completed    bool       `json:"completed"`
-	StartedAt    *time.Time `json:"started_at"`
-}
-
 type Session struct {
 	Token      string             `json:"token"`
 	IpAddress  string             `json:"ip_address"`