Deploy SearxNG on ai-server-4080 for chat_backend grounded search (#10) (#11)

## Summary
- Closes [#10](#10).
- Deploys **SearxNG** on **ai-server-4080** (`10.0.0.128`) for [chat_backend#62](ai_ml_operations/chat_backend#62) / [PR #65](ai_ml_operations/chat_backend#65) grounded search.
- New `roles/searxng/` (compose + JSON-enabled `settings.yml`), gated by `searxng_stack: true`, wired into `site.yml`.
- **Host port 8088** (not 8080 — that is `dta_webapp` on this host). UFW allows `10.0.0.0/24` → `8088/tcp` only.

## Ops after merge

```bash
./scripts/provision.sh ai-server-4080
# or targeted:
ansible-playbook playbooks/site.yml --limit ai-server-4080 --tags never  # full site play includes searxng when searxng_stack
```

Then set in `chat_backend_prod.env` / `chat_backend_beta.env`:

```text
SEARCH_PROVIDER=searxng
SEARCH_FAILOVER_PROVIDER=ddgs
SEARXNG_BASE_URL=http://10.0.0.128:8088
```

Smoke test from any app host:

```bash
curl -sG 'http://10.0.0.128:8088/search' --data-urlencode 'q=test' -d 'format=json' | head
```

## Test plan
- [ ] Provision ai-server-4080; confirm `docker ps` shows `searxng`
- [ ] Confirm `:8088` responds with JSON; `:8080` still serves dta_webapp
- [ ] Confirm UFW rule is LAN-only
- [ ] From adama/roslin container network, curl SearxNG succeeds
- [ ] Update chat_backend secrets to `:8088` and redeploy betaReviewed-on: #11

scripts/provision.sh

@@ -15,7 +15,8 @@ Provision server(s) with site.yml.
 
 Applies: common, ufw, docker, nodejs, gitea-key, tianji, alloy (every host).
 On ai-server-4080 also: observability (Loki + Prometheus + Grafana) when
-observability_stack is true in host_vars.
+observability_stack is true, and SearxNG when searxng_stack is true
+(chat_backend grounded search on :8088).
 
   HOST              Optional. Limit to one host: adama, roslin, or ai-server-4080.
                     Omit to run against all webservers.
@@ -31,7 +32,7 @@ Examples:
   $(basename "$0") adama --check    # dry run on adama only
   $(basename "$0") adama            # provision adama (includes Alloy)
   $(basename "$0") adama --ask-pass  # first SSH login before ssh-copy-id
-  $(basename "$0") ai-server-4080   # control node + Loki/Prometheus/Grafana
+  $(basename "$0") ai-server-4080   # control node + Loki/Prometheus/Grafana + SearxNG
   $(basename "$0")                  # provision all hosts
 EOF
 }