feat(events): add execution-backed event pipeline

[psteinroe]

Make custom events durable by representing each emission as an internal execution and fanning it out asynchronously through a hidden dispatcher worker. This removes shared-inbox and per-worker event polling overhead while preserving typed event APIs.

PostgreSQL-native fan-out

Custom-event filters compile into normalized matcher tables. Claim-fenced dispatch uses indexed candidate discovery plus complete residual verification, deduplicates subscriptions, and atomically writes destination executions with a completion marker. Destination uniqueness makes retries idempotent, while per-source validation failures do not roll back valid peers in the same batch.

Application-owned database triggers

Pgconductor no longer creates, synchronizes, or manages triggers on application tables. Applications that emit events from PostgreSQL triggers own those triggers and can call pgconductor.emit_event() transactionally, so source changes and event persistence still commit or roll back together.

Worker lifecycle and bounded matching

The hidden dispatcher reuses the normal claim, execution, retry, and result-flush lifecycle with an initial batch size of 10. Registration precedes dispatcher polling, recursive drain passes process newly emitted work, and registration/dispatch enforce documented limits for subscriptions, filters, scalar event fields, and destinations.

Refs #14