fix(#189): add composite database index for job_events table

Add composite index [jobId, timestamp] to improve query performance
for the most common job_events access patterns.

Changes:
- Add @@index([jobId, timestamp]) to JobEvent model in schema.prisma
- Create migration 20260202122655_add_job_events_composite_index
- Add performance tests to validate index effectiveness
- Document index design rationale in scratchpad
- Fix lint errors in api-key.guard, herald.service, runner-jobs.service

Rationale:
The composite index [jobId, timestamp] optimizes the dominant query
pattern used across all services:
- JobEventsService.getEventsByJobId (WHERE jobId, ORDER BY timestamp)
- RunnerJobsService.streamEvents (WHERE jobId + timestamp range)
- RunnerJobsService.findOne (implicit jobId filter + timestamp order)

This index provides:
- Fast filtering by jobId (highly selective)
- Efficient timestamp-based ordering
- Optimal support for timestamp range queries
- Backward compatibility with jobId-only queries

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/herald/herald.service.ts

@@ -101,10 +101,7 @@ export class HeraldService {
       this.logger.debug(`Broadcasted event ${event.type} for job ${jobId} to thread ${threadId}`);
     } catch (error) {
       // Log the error with full context for debugging
-      this.logger.error(
-        `Failed to broadcast event ${event.type} for job ${jobId}:`,
-        error
-      );
+      this.logger.error(`Failed to broadcast event ${event.type} for job ${jobId}:`, error);
 
       // Re-throw the error so callers can handle it appropriately
       // This enables proper error tracking, retry logic, and alerting