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)

pkg/diunwebhook/migrations/postgres/0001_initial_schema.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS tag_assignments;
+DROP TABLE IF EXISTS tags;
+DROP TABLE IF EXISTS updates;