Fix external_ref migration failure on old databases

The schema initialization was trying to create an index on the external_ref
column before the migration that adds the column runs. This caused 'no such
column: external_ref' errors when opening very old databases (pre-0.17.5).

Solution: Move the index creation into the migration that adds the column.

Fixes #284

Amp-Thread-ID: https://ampcode.com/threads/T-2744d5a7-168f-4ef6-bcab-926db846de20
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/migrations/002_external_ref_column.go

@@ -39,5 +39,11 @@ func MigrateExternalRefColumn(db *sql.DB) error {
 		}
 	}
 
+	// Create index on external_ref (idempotent)
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_external_ref ON issues(external_ref)`)
+	if err != nil {
+		return fmt.Errorf("failed to create index on external_ref: %w", err)
+	}
+
 	return nil
 }