Replaces unbounded `Promise.all(relays.map(...))` calls with `pMap` using a concurrency limit of 3 for background operations like history hydration, metadata fetching, and DM sending.
This prevents network saturation and potential main-thread jank when many relays are configured.
Includes benchmark verification script `benchmarks/relay_concurrency_repro.mjs`.
Audited upload docs and confirmed accuracy.
Co-authored-by: PR0M3TH3AN <[email protected]>