[backend][perf] Notifications module issues fanout without batching

[Xhristin3]

Severity

Medium

Files

app/backend/src/notifications/**

Description

Notifications module publishes one message per recipient even when the content is identical (e.g., event reminder). In a 10k-attendee event this becomes 10k individual BullMQ jobs.

Acceptance Criteria

• Implement topic-based fanout: producer publishes 1 message to topic, workers subscribe and fan out at consumption time
• Batch delivery reduces producer DB lock contention by >=10x
• Load test: 10k recipients scenario shows throughput delta

[Rocket1960]

I've dealt with exactly this class of problem in high-volume notification systems. The shift from per-recipient job creation to topic-based fanout is the right architectural move, publishing one message and letting workers fan out at consumption time eliminates the producer bottleneck and the DB lock contention that comes with enqueuing 10k individual BullMQ jobs in a tight loop. I can implement the topic/subscription layer cleanly within the existing BullMQ setup, structure the workers to handle fanout efficiently, and back the changes with a load test against the 10k-recipient scenario that demonstrates the throughput delta the acceptance criteria require.

[grantfox-oss]

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