bug(ansible): backend.env.j2 AUTOBOT_CHROMADB_HOST uses ai_stack_host fallback, not backend_ai_stack_host — WSL2 gets wrong IP

[mrveiss]

Problem

In `backend.env.j2`:

```jinja2
AUTOBOT_CHROMADB_HOST={{ backend_chromadb_host | default(ai_stack_host | default('127.0.0.1')) }}
```

The fallback chain uses `ai_stack_host`, not `backend_ai_stack_host`. The WSL2 auto-detection added in #3503/#3516 only sets `backend_ai_stack_host`. So on a co-located WSL2 deployment where `backend_chromadb_host` is not explicitly set in inventory, `AUTOBOT_CHROMADB_HOST` still resolves to `127.0.0.1` — the wrong loopback IP on WSL2 mirrored networking — causing ECONNREFUSED to ChromaDB.

Discovered During

#3503 / PR #3516 implementation — noticed the fallback variable mismatch in the template.

Fix

Change the fallback in `backend.env.j2` to use `backend_ai_stack_host`:

```jinja2
AUTOBOT_CHROMADB_HOST={{ backend_chromadb_host | default(backend_ai_stack_host | default('127.0.0.1')) }}
```

The WSL2 detection task already sets `backend_ai_stack_host` correctly — this just wires ChromaDB to use the same resolved value.

Affected Files

• `autobot-slm-backend/ansible/roles/backend/templates/backend.env.j2` (line 52)