feat: add websocket pooling and streaming optimizations

[mimijuwonlo-commits]

Summary

• add backend WebSocket connection pooling with outbound message batching and metrics
• add streaming response helpers/routes for large dataset exports with batch-size tuning
• add read replica config, failover routing, optional Terraform RDS replicas, and lag alarms
• centralize API compression middleware and update performance docs/env examples

Tests

• Added unit coverage for WebSocket pooling, streaming helpers, compression decisions, and read replica routing.
• Not run locally: dependency install is blocked because package-lock files are out of sync and npm fetch failed with EACCES; Terraform CLI is not installed in this environment.

Closes #741.
Closes #743.
Closes #744.
Closes #734.

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