bitcoin-knots, fulcrum, datum-gateway: add and use the socket_proxy role

Three near-identical hosts: edge plays become one role plus three short
calls. 183 lines removed, 34 added, plus a 111-line role.

Verified before touching any playbook: all six live units on vipy reproduced
byte-identically. Then --limit edge --check per playbook - bitcoin-knots and
fulcrum changed=0; datum-gateway changed=2, both attributable to the already
known caddy_site comment line and the Reload caddy handler it triggers.
The 6 units and 14 Caddy files on the hosts are byte-identical afterwards.

PLAN_4 claimed these three plays had "no behavioural drift at all". That was
wrong - it came from a diff truncated by head -60. The live bitcoin-p2p-proxy
units carry four settings this playbook never wrote:

  .socket   Documentation=, FreeBind=true
  .service  Documentation=, TimeoutStopSec=5,
            StandardOutput=journal, StandardError=journal

FreeBind is the one that matters: it lets the socket bind to an address that
is not up yet, so without it the socket can fail to start on boot. Running
the bitcoin-knots playbook would have stripped it. Same class of hazard as
headscale. The role expresses all four; bitcoin-p2p is the only caller that
passes any.

Also: UFW treats the rule comment as part of the rule. datum-stratum's live
comment is "DATUM Gateway Stratum public access" but the role's derived
default produced "DATUM Stratum public access", which rewrote the rule.
Caught in the dry-run; datum now passes the comment explicitly.

Two deliberate differences from the original, both documented in the README:
ignore_errors: yes on the upstream check became failed_when: false, and the
handler restarts the .socket, which drops connections open through it - it
fires only when a unit file actually changes.

The inert Uptime Kuma TCP monitor blocks stay in the playbooks rather than
being pulled into a new role (12/12/18 guarded tasks).

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

ansible/services/datum-gateway/deploy_datum_gateway_playbook.yml

@@ -663,62 +663,17 @@
     uptime_kuma_api_url: "https://{{ subdomains.uptime_kuma }}.{{ root_domain }}"
 
   tasks:
-    - name: Create Stratum proxy socket unit
-      copy:
-        dest: /etc/systemd/system/datum-stratum-proxy.socket
-        content: |
-          [Unit]
-          Description=DATUM Stratum Proxy Socket
-
-          [Socket]
-          ListenStream={{ datum_gateway_stratum_port }}
-
-          [Install]
-          WantedBy=sockets.target
-        owner: root
-        group: root
-        mode: "0644"
-      notify: Restart datum-stratum-proxy socket
-
-    - name: Create Stratum proxy service unit
-      copy:
-        dest: /etc/systemd/system/datum-stratum-proxy.service
-        content: |
-          [Unit]
-          Description=DATUM Stratum Proxy to {{ datum_tailscale_hostname }}
-          Requires=datum-stratum-proxy.socket
-          After=network.target
-
-          [Service]
-          Type=notify
-          ExecStart=/lib/systemd/systemd-socket-proxyd {{ datum_tailscale_hostname }}:{{ datum_gateway_stratum_port }}
-        owner: root
-        group: root
-        mode: "0644"
-
-    - name: Reload systemd daemon
-      systemd:
-        daemon_reload: yes
-
-    - name: Enable and start Stratum proxy socket
-      systemd:
-        name: datum-stratum-proxy.socket
-        enabled: yes
-        state: started
-
-    - name: Allow Stratum port through UFW
-      ufw:
-        rule: allow
-        port: "{{ datum_gateway_stratum_port | string }}"
-        proto: tcp
-        comment: "DATUM Gateway Stratum public access"
-
-    - name: Verify connectivity to knots-box Stratum via Tailscale
-      wait_for:
-        host: "{{ datum_tailscale_hostname }}"
-        port: "{{ datum_gateway_stratum_port }}"
-        timeout: 10
-      ignore_errors: yes
+    - name: Expose the DATUM Stratum port through a socket proxy
+      ansible.builtin.include_role:
+        name: socket_proxy
+      vars:
+        socket_proxy_name: datum-stratum
+        socket_proxy_description: "DATUM Stratum"
+        socket_proxy_listen_port: "{{ datum_gateway_stratum_port }}"
+        socket_proxy_upstream_host: "{{ datum_tailscale_hostname }}"
+        # Matches the UFW comment already on vipy; the derived default would
+        # have said "DATUM Stratum" and rewritten the rule.
+        socket_proxy_ufw_comment: "DATUM Gateway Stratum public access"
 
     - name: Display public Stratum endpoint
       when: uptime_kuma_enabled | default(false)
@@ -845,8 +800,3 @@
         - /tmp/setup_datum_stratum_tcp_monitor.py
         - /tmp/ansible_datum_stratum_config.yml
 
-  handlers:
-    - name: Restart datum-stratum-proxy socket
-      systemd:
-        name: datum-stratum-proxy.socket
-        state: restarted