chore: upgrade etl and adopt memory-based batching

[psteinroe]

• Bump etl and etl-postgres to the latest upstream revision and add sysinfo for memory-aware batch sizing
• Replace count-based batch config with memory_budget_ratio and max_fill_ms across code, tests, and docs
• Update replay batching to use byte budgets, emit on errors, and log memory-based batch settings
• Remove local ETL migrations and switch PostgresStore::new call sites to async initialization