feat(03-01): add pgx/v5, PostgreSQL migrations, rename RunMigrations to RunSQLiteMigrations

- Add github.com/jackc/pgx/v5 v5.9.1 dependency
- Add golang-migrate pgx/v5 driver
- Create migrations/postgres/0001_initial_schema.up.sql with SERIAL PRIMARY KEY
- Create migrations/postgres/0001_initial_schema.down.sql
- Rename RunMigrations to RunSQLiteMigrations in migrate.go
- Add RunPostgresMigrations with pgxmigrate driver and 'pgx5' name
- Update export_test.go to use RunSQLiteMigrations (go vet compliance)

go.mod

@@ -10,11 +10,18 @@ require (
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
+	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/pgx/v5 v5.9.1 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect
+	golang.org/x/sync v0.18.0 // indirect
 	golang.org/x/sys v0.38.0 // indirect
+	golang.org/x/text v0.31.0 // indirect
 	modernc.org/libc v1.67.6 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect