log_sql: true mode — emit the exact transfer SQL for debugging

[sncalvo]

Problem

When imports misbehave in prod ("why did 10 rows silently drop?"), the first thing anyone wants is the exact SQL the gem ran. Right now you have to attach a debugger or subscribe to AR's notification channel.

Proposal

StagingTable.stage(User, log_sql: true) { |s| s.insert(rows) }
# → [staging_table] SQL: CREATE TEMP TABLE staging_users_abc123 ...
# → [staging_table] SQL: INSERT INTO staging_users_abc123 (...) VALUES ...
# → [staging_table] SQL: INSERT INTO users (...) SELECT ... FROM staging_users_abc123 ON CONFLICT ...

• Emits via the configured logger
• Truncates very long VALUES lists (...)
• Global enable via config

Acceptance criteria

• log_sql: true per-call works
• config.log_sql = true enables globally
• Sensitive values masked if config.mask_columns = [:password_digest, :ssn]
• Pairs with the logger-integration issue