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

.github/workflows/platform-backend-ci.yml

@@ -41,13 +41,18 @@ jobs:
         ports:
           - 6379:6379
       rabbitmq:
-        image: rabbitmq:3.12-management
+        image: rabbitmq:4.1.4
         ports:
           - 5672:5672
-          - 15672:15672
         env:
           RABBITMQ_DEFAULT_USER: ${{ env.RABBITMQ_DEFAULT_USER }}
           RABBITMQ_DEFAULT_PASS: ${{ env.RABBITMQ_DEFAULT_PASS }}
+        options: >-
+          --health-cmd "rabbitmq-diagnostics -q ping"
+          --health-interval 30s
+          --health-timeout 10s
+          --health-retries 5
+          --health-start-period 10s
       clamav:
         image: clamav/clamav-debian:latest
         ports: