Replace MySQL driver with pgx/v5 and pgxpool:
- Update sqlc to use postgresql engine
- Convert query.sql to PostgreSQL syntax ($1 params, CASE WHEN,
ANY() arrays)
- Replace sql.DB with pgxpool.Pool throughout
- Change nullable types from sql.Null* to pgtype.*
- Update ID types from uint32 to int64 for PostgreSQL compatibility
- Delete MySQL-specific dynamic_connect.go
- Add opentelemetry.gowrap template for tracing
