ansible: add site.yml, and rename the monitoring group off the host's name

site.yml is a TABLE OF CONTENTS, not a second source of truth. It is 25
import_playbook: lines and comments - no `hosts:`, no `roles:`. Which hosts get
what stays on the `hosts:` line inside each playbook, exactly where it already
was; nothing moved. Every role is already wrapped in a thin playbook carrying
its own `hosts:` line, so there is no roles-vs-playbooks split to reconcile:
from here everything is a playbook.

What it buys:

  What runs on a host?  ansible-playbook site.yml --limit <host> --list-hosts
  Who gets thing Y?     the `hosts:` line in Y's own playbook
  What is a host?       ansible-inventory --graph

Note --list-hosts, not --list-tasks: the latter prints every play regardless of
--limit, so it will happily show you the bitcoin play under memos-box.

Nine playbooks are deliberately excluded and the file names every one with a
reason, so it accounts for all of them: the three infra/4xx monitoring plays
(still assert on the removed Uptime Kuma credentials and fail immediately),
910_docker (says `hosts: managed`, but Docker is on 5 of 11 managed hosts and
those 5 are exactly the ones that need it - running it installs Docker on the
Bitcoin node and the hypervisor), two nodito one-shots, the Kuma notification
setup, and two deliberate manual actions.

Writing it surfaced an inventory collision. There is a HOST named `monitoring`
in [vps] AND a group [monitoring], so Ansible warned and resolved `hosts:
monitoring` to the host:

  [WARNING]: Found both group and host with same name: monitoring

The group is renamed to [observability]; the host keeps its name. [caddy:children]
and the two ntfy playbooks follow. Behaviour is unchanged - `hosts: monitoring`
already resolved to the host - but the ambiguity is gone and the warning with it.

Verified: inventory graph is warning-free, site.yml passes --syntax-check, and
per-host play counts are identical before and after the rename.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/services/ntfy/setup_ntfy_uptime_kuma_notification.yml

@@ -12,7 +12,7 @@
 # What was being monitored: archive/uptime_kuma/MONITORS.md
 # ═════════════════════════════════════════════════════════════════════════════
 - name: Setup ntfy as Uptime Kuma Notification Channel
-  hosts: monitoring
+  hosts: observability
   become: no
   vars_files:
     - ./ntfy_vars.yml