feat(table): add optional PG NOTIFY trigger for outbox tables

[OpenSourceAlchemist]

Description

Add an optional PostgreSQL NOTIFY trigger that signals when new events are
written to the outbox table. This allows external subscribers to react to
outbox events in near real-time without polling.

Changes

• New NotifyTrigger class creating a pg_notify function and
  statement-level AFTER INSERT trigger on the outbox table
• Opt-in with notify: true option on pgt_outbox_setup
• Channel defaults to <outbox_table>_notifications, overridable via notify_channel
• Follows pgt_outbox_* naming convention for generated objects
• Add --notify and --notify-channel CLI flags to outboxify

API

DB.pgt_outbox_setup(:accounts,
  notify: true,
  notify_channel: 'accounts_outbox_notifications'  # default
)

Closes #28