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/prisma/schema.prisma

@@ -1209,5 +1209,6 @@ model JobEvent {
   @@index([stepId])
   @@index([timestamp])
   @@index([type])
+  @@index([jobId, timestamp])
   @@map("job_events")
 }