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

roles/searxng/tasks/main.yml

@@ -0,0 +1,64 @@
+---
+# SearxNG JSON search API for chat_backend grounded retrieval.
+# Enabled on ai-server-4080 via searxng_stack: true (issue #10).
+
+- name: searxng | ensure project directory
+  ansible.builtin.file:
+    path: "{{ searxng_dir }}"
+    state: directory
+    owner: "{{ admin_user }}"
+    group: "{{ admin_user }}"
+    mode: "0750"
+
+- name: searxng | settings.yml
+  ansible.builtin.template:
+    src: settings.yml.j2
+    dest: "{{ searxng_dir }}/settings.yml"
+    owner: "{{ admin_user }}"
+    group: "{{ admin_user }}"
+    mode: "0640"
+  register: _searxng_settings
+
+- name: searxng | compose file
+  ansible.builtin.template:
+    src: docker-compose.yml.j2
+    dest: "{{ searxng_dir }}/docker-compose.yml"
+    owner: "{{ admin_user }}"
+    group: "{{ admin_user }}"
+    mode: "0644"
+  register: _searxng_compose
+
+- name: searxng | allow JSON API from LAN
+  community.general.ufw:
+    rule: allow
+    port: "{{ searxng_host_port }}"
+    proto: tcp
+    from_ip: "{{ searxng_ufw_from }}"
+    comment: SearxNG for chat_backend grounded search
+
+- name: searxng | start stack
+  ansible.builtin.command:
+    cmd: >-
+      docker compose up -d --remove-orphans
+      {{ '--force-recreate' if (
+        _searxng_compose is changed
+        or _searxng_settings is changed
+      ) else '' }}
+    chdir: "{{ searxng_dir }}"
+  become: true
+  become_user: "{{ admin_user }}"
+  register: _searxng_up
+  changed_when: >-
+    _searxng_up.rc == 0 and (
+      'Started' in (_searxng_up.stdout | default(''))
+      or 'Recreated' in (_searxng_up.stdout | default(''))
+      or 'Created' in (_searxng_up.stdout | default(''))
+      or _searxng_compose is changed
+      or _searxng_settings is changed
+    )
+  failed_when: _searxng_up.rc != 0
+
+- name: searxng | show compose failure output
+  ansible.builtin.debug:
+    msg: "{{ _searxng_up.stderr_lines | default(_searxng_up.stdout_lines) }}"
+  when: _searxng_up is failed