ci(backend): Upgrade RabbitMQ from 3.12 (EOL) to 4.1.4 (#12118)

## Summary
Upgrades RabbitMQ from the end-of-life `rabbitmq:3.12-management` to
`rabbitmq:4.1.4`, aligning CI, local dev, and e2e testing with
production.

## Changes

### CI Workflow (`.github/workflows/platform-backend-ci.yml`)
- **Image:** `rabbitmq:3.12-management` → `rabbitmq:4.1.4`
- **Port:** Removed 15672 (management UI) — not used
- **Health check:** Added to prevent flaky tests from race conditions
during startup

### Docker Compose (`docker-compose.platform.yml`,
`docker-compose.test.yaml`)
- **Image:** `rabbitmq:management` → `rabbitmq:4.1.4`
- **Port:** Removed 15672 (management UI) — not used

## Why
- RabbitMQ 3.12 is EOL
- We don't use the management interface, so `-management` variant is
unnecessary
- CI and local dev/e2e should match production (4.1.4)

## Testing
CI validates that backend tests pass against RabbitMQ 4.1.4 on Python
3.11, 3.12, and 3.13.

---
Closes SECRT-1703