Feature: Plugin SDK for Custom Event Enrichment and Integrations

## Problem
Teams often need domain-specific metadata (ticket IDs, environments, owners, service names) attached to events, but the ingestion pipeline is currently fixed.

## Proposal
Create a **Plugin SDK** that lets users register server-side plugins to enrich events and hook into lifecycle callbacks.

## Scope
- Plugin lifecycle: load, validate, enable/disable, version
- Hooks: `beforePersist(event)`, `afterPersist(event, entities)`, `beforeBroadcast(message)`
- Safe plugin sandboxing and timeout guards
- Plugin diagnostics UI (health, last execution, failures)

## Example Use Cases
- Add Jira ticket extraction from summaries
- Tag events by environment from cwd/path
- Forward selected events to external systems

## Technical Notes
- Define stable plugin API contract with TypeScript types
- Isolate plugin errors so ingestion never fails hard
- Add plugin registry table for metadata and state

## Acceptance Criteria
- [ ] Plugins can be installed and toggled without server code edits
- [ ] Plugin failure does not block core event ingestion
- [ ] Plugin execution is observable (latency, error count)
- [ ] Enriched fields are persisted and queryable
- [ ] SDK docs include a working example plugin