fix(ansible/redis): redis-stack-server unavailable on Ubuntu 24.04 (noble) — pin Redis repo to jammy fallback #7178

Symptom

Provision Phase 3: Data Layer fails on DGX Spark (Ubuntu 24.04, ARM64) with:

```
TASK [redis : Redis | Install Redis Stack] *************************************
fatal: [00-SLM-Manager]: FAILED! => {
"changed": false,
"msg": "No package matching 'redis-stack-server' is available"
}
```

Cause

`autobot-slm-backend/ansible/roles/redis/tasks/main.yml:24` adds the Redis repo using `{{ ansible_distribution_release }}` — which evaluates to `noble` on Ubuntu 24.04. But Redis only ships `redis-stack-server` for older suites:

```
jammy amd64: redis-stack-server present (43 entries)
jammy arm64: redis-stack-server present (42 entries)
noble amd64: redis-stack-server ABSENT
noble arm64: redis-stack-server ABSENT
bookworm: redis-stack-server ABSENT
```

This affects EVERY Ubuntu 24.04 install (DGX Spark + any Hyper-V VM upgraded from 22.04 to 24.04). Not arm64-specific.

Fix

Pin the Redis repo suite to `jammy` for distributions where `redis-stack-server` isn't published. `jammy` packages run fine on noble (same glibc family, statically-built redis modules).

```yaml

• name: "Redis | Determine Redis Stack repo suite"
  ansible.builtin.set_fact:
  redis_stack_suite: "{{ 'jammy' if ansible_distribution_release == 'noble' else ansible_distribution_release }}"

• name: "Redis | Add Redis Stack repository"
  apt_repository:
  repo: "deb https://packages.redis.io/deb {{ redis_stack_suite }} main"
  state: present
  filename: redis
  ```

Reproduced

DGX Spark 192.168.88.96 (ARM64, noble), 2026-05-07T~14:00Z.

Acceptance criteria

• Provision Phase 3 "Install Redis Stack" succeeds on Ubuntu 24.04 (noble) on both x86_64 and arm64
• No regression on Ubuntu 22.04 (jammy) — the existing default
• Comment in the role explains why the suite is overridden for noble