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/roles/socket_proxy/tasks/main.yml

@@ -0,0 +1,54 @@
+---
+- name: Assert socket_proxy parameters are sane
+  ansible.builtin.assert:
+    that:
+      - socket_proxy_name | length > 0
+      - socket_proxy_description | length > 0
+      - socket_proxy_listen_port | int > 0
+      - socket_proxy_upstream_host | length > 0
+    fail_msg: >-
+      socket_proxy: '{{ socket_proxy_name | default("<unnamed>") }}' needs a name,
+      a description, a listen port and an upstream host.
+    quiet: true
+
+- name: "Create the {{ socket_proxy_name }}-proxy socket unit"
+  ansible.builtin.template:
+    src: proxy.socket.j2
+    dest: "/etc/systemd/system/{{ socket_proxy_name }}-proxy.socket"
+    owner: root
+    group: root
+    mode: '0644'
+  notify: Restart socket proxy
+
+- name: "Create the {{ socket_proxy_name }}-proxy service unit"
+  ansible.builtin.template:
+    src: proxy.service.j2
+    dest: "/etc/systemd/system/{{ socket_proxy_name }}-proxy.service"
+    owner: root
+    group: root
+    mode: '0644'
+  notify: Restart socket proxy
+
+- name: "Enable and start the {{ socket_proxy_name }}-proxy socket"
+  ansible.builtin.systemd:
+    name: "{{ socket_proxy_name }}-proxy.socket"
+    enabled: yes
+    state: started
+    daemon_reload: yes
+
+- name: "Allow the {{ socket_proxy_name }} port through UFW"
+  community.general.ufw:
+    rule: allow
+    port: "{{ socket_proxy_listen_port | string }}"
+    proto: "{{ socket_proxy_ufw_proto }}"
+    comment: "{{ socket_proxy_ufw_comment | default(socket_proxy_description ~ ' public access', true) }}"
+
+# Reachability of the upstream over Tailscale. Deliberately non-fatal: the proxy
+# is still correctly configured if the backend happens to be down, and this is
+# the one check that depends on another machine being up.
+- name: "Verify {{ socket_proxy_upstream_host }} is reachable over Tailscale"
+  ansible.builtin.wait_for:
+    host: "{{ socket_proxy_upstream_host }}"
+    port: "{{ socket_proxy_upstream_port | default(socket_proxy_listen_port, true) }}"
+    timeout: 10
+  failed_when: false