Materialized view refresh helper (after_transfer: refresh_matview(:name))

[sncalvo]

Problem

Many Postgres apps have materialized views on top of the tables StagingTable writes to. After a transfer, the matview is stale until someone runs REFRESH MATERIALIZED VIEW. Users currently do this manually in an after_transfer callback.

Proposal

StagingTable.stage(User,
  after_transfer: StagingTable::Helpers.refresh_matview(:user_stats, concurrently: true)
) { |s| s.insert(rows) }

• Works with CONCURRENTLY (requires a unique index on the matview — document)
• Multiple matviews: pass an array

Acceptance criteria

• Postgres-only helper; clear error on other adapters
• Supports concurrently: true/false
• Docs reference the matview unique-index prerequisite