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/templates/proxy.socket.j2

@@ -0,0 +1,14 @@
+[Unit]
+Description={{ socket_proxy_description }} Proxy Socket
+{% if socket_proxy_documentation %}
+Documentation={{ socket_proxy_documentation }}
+{% endif %}
+
+[Socket]
+ListenStream={{ socket_proxy_listen_port }}
+{% if socket_proxy_free_bind %}
+FreeBind=true
+{% endif %}
+
+[Install]
+WantedBy=sockets.target