Wip/perf diagnostics

[danoswaltCL]

when PERF_DIAG_ENABLED is true, this will give us a heartbeat every 10 seconds per instance some diagnostic info about event loop and db pool, so we can try and catch what's really going on inside the instances.

so will want to add these to param store:

TYPEORM_MAX_CONNECTION_POOL_SIZE=20
PERF_DIAG_ENABLED=true

{"tag":"perfdiag","ts":"2026-09-29T19:32:22.664Z","instanceId":"b2d945d5","eventLoopBusyPct":39.3,"eventLoopDelayMs":{"p50":0.3,"p99":12,"max":16.6},"garbageCollection":{"major":{"count":1,"totalMs":4.5,"maxMs":4.5},"minor":{"count":272,"totalMs":200.5,"maxMs":4.4}},"dbPool":{"maxWaitingForConnection":0,"maxConnectionsInUse":8,"poolSize":10},"heapUsedMb":164}

[Copilot]

Copilot review overview

🟡 Changes recommended

Non-minor GC events are incorrectly reported as minor collections, producing misleading diagnostics.

Review effort: Balanced
Findings: 1

Open (1)

• Misclassifies incremental and weak callback GC events as minor · New

What changed in this PR

Adds opt-in backend diagnostics for investigating request latency and database pool saturation.

Changes:

• Emits 10-second JSON heartbeats with event-loop, GC, heap, and PostgreSQL pool metrics.
• Enables diagnostics through PERF_DIAG_ENABLED.
• Adds unit coverage for reporting and shutdown behavior.

File	Description
packages/​backend/​src/​env.ts	Adds the diagnostics feature flag.
packages/​backend/​src/​lib/​perfDiagnostics.ts	Collects and emits performance metrics.
packages/​backend/​src/​loaders/​typeormLoader.ts	Starts diagnostics after database initialization.
packages/​backend/​test/​unit/​lib/​perfDiagnostics.test.ts	Tests output, missing pools, and stopping.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The diagnostics are opt-in, failure-isolated, tested, and introduce no identified correctness issues.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Misclassifies incremental and weak callback GC events as minor