[Backend] Add read replicas and PgBouncer pooling

[Marvellous805]

Closes #1325

Changes

• Configure Prisma read replicas from DATABASE_READ_REPLICA_URL; writes and transactions remain on the primary.
• Route stream listing, stream events, event history, and summary reads through replica-aware fallback with a short outage cooldown.
• Set pool defaults to 20 connections, 10-second idle timeout, and 2-second connection timeout.
• Add a PgBouncer transaction-pooling sidecar to Docker Compose, capped at 20 PostgreSQL connections.

Validation

• git diff --check
• Parsed backend/package.json and package-lock.json as JSON.
• Test suite and 1,000-request load test were not run in this environment.