datum-gateway: convert to a role, de-Uptime-Kuma the health check

802-line playbook becomes 68 lines (three plays: the role, the Caddy dashboard,
the Stratum socket proxy) plus a 345-line role. datum_gateway_vars.yml is
deleted; its content is the role's defaults.

Verified after a real run with zero miners connected: datum-gateway restarted
cleanly onto the reformatted config, deployed config.json semantically identical
to what was there (pool_address bc1qvrj3g84..., pool_pass_* false, ports
unchanged), health check timer firing, and the Knots side untouched - bitcoind
still up since 2026-08-19 with blocknotify intact.

TWO PIECES OF DRIFT WHERE THE NODE WAS RIGHT, both confirmed with the operator:

- datum_mining_address: the vault held bc1qdse9dsg... while the node had been
  mining to bc1qvrj3g... since 2026-08-08. This is WHERE BLOCK REWARDS ARE PAID.
  And unlike fulcrum and bitcoin-knots, the `Restart datum-gateway` handler here
  was never gated, so the stale value would have applied immediately rather than
  sitting inert on disk.
- pool_pass_workers / pool_pass_full_users: false on the node, true in the vars
  file.

Both corrected in the vault and role defaults with notes recording why.

Comparing this config needs semantics, not text: the live file is single-line
JSON and the template renders pretty-printed, so a textual diff is pure noise.
Rendering it and comparing parsed JSON is what surfaced both differences.

config.json carries bitcoind.rpcpassword and api.admin_password, and --diff
prints rendered content - so `--check --diff` put them on the terminal. The task
now sets diff: false by default (-e datum_reveal_config=true to opt in). Those
two should be rotated.

I also mis-reported pool_pass_workers/pool_pass_full_users as exposed credentials
because my masking matched "pass" in the key name. They are BOOLEANS, and
mining.pool_address is a Bitcoin address, public by nature. Only the two real
passwords above were exposed.

`Configure cmake build` and `Compile datum_gateway` are bare command: tasks with
no changed_when, so they recompile on every run. The build is reproducible -
Install datum_gateway binary sees identical content and leaves the installed
binary's timestamp alone - but it is wasted work each time. Documented as the
idempotent floor.

Ownership parity checked mechanically against `git show HEAD:` keyed by task
name: 7/7 match, 9 Kuma tasks dropped.

This completes Plan 6 Stage 2: all six services in the list are roles.

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

ansible/roles/datum_gateway/tasks/configure.yml

@@ -0,0 +1,25 @@
+---
+# Ownership copied verbatim from the playbook this replaces and verified
+# mechanically against `git show HEAD:`.
+- name: Write DATUM Gateway config.json
+  ansible.builtin.template:
+    src: config.json.j2
+    dest: "{{ datum_gateway_config_dir }}/config.json"
+    owner: "{{ datum_gateway_user }}"
+    group: "{{ datum_gateway_group }}"
+    mode: '0640'
+  # config.json carries the bitcoind RPC password, the API admin password and
+  # the pool passwords. `--diff` prints rendered content, so running with --diff
+  # put all of them on the terminal and into any log capturing it. Suppressed by
+  # default; pass -e datum_reveal_config=true when you genuinely need the diff.
+  diff: "{{ datum_reveal_config | default(false) | bool }}"
+  notify: Restart datum-gateway
+
+- name: Create datum-gateway systemd service
+  ansible.builtin.template:
+    src: datum-gateway.service.j2
+    dest: /etc/systemd/system/datum-gateway.service
+    owner: root
+    group: root
+    mode: '0644'
+  notify: Restart datum-gateway

ansible/roles/datum_gateway/tasks/healthcheck.yml

@@ -0,0 +1,50 @@
+---
+# Everything here answers "is DATUM Gateway healthy" and records the answer. The
+# Uptime Kuma specifics that used to follow — an embedded Python script creating
+# monitors over the API, a /tmp credentials file, a push-URL file read back and
+# parsed, and a systemd Environment= rewrite — are gone. Where it reports is now
+# one variable, healthcheck_push_url.
+- name: Create DATUM Gateway health check script
+  ansible.builtin.template:
+    src: healthcheck.sh.j2
+    dest: /usr/local/bin/datum-gateway-healthcheck-push.sh
+    owner: root
+    group: root
+    mode: '0755'
+    validate: "bash -n %s"
+
+- name: Create datum-gateway health check systemd service
+  ansible.builtin.template:
+    src: healthcheck.service.j2
+    dest: /etc/systemd/system/datum-gateway-healthcheck.service
+    owner: root
+    group: root
+    mode: '0644'
+  notify: Restart datum-gateway health check timer
+
+- name: Create datum-gateway health check systemd timer
+  ansible.builtin.template:
+    src: healthcheck.timer.j2
+    dest: /etc/systemd/system/datum-gateway-healthcheck.timer
+    owner: root
+    group: root
+    mode: '0644'
+  notify: Restart datum-gateway health check timer
+
+- name: Reload systemd daemon after health check units
+  systemd:
+    daemon_reload: yes
+
+# Ungated: enabling a timer is deployment, not monitoring.
+- name: Enable and restart the datum-gateway health check timer
+  systemd:
+    name: datum-gateway-healthcheck.timer
+    enabled: yes
+    state: restarted
+    daemon_reload: yes
+
+# Arms the timer and smoke-tests the check. See roles/bitcoin_knots/README.md for
+# why restarting the timer alone is not enough with OnBootSec + OnUnitActiveSec.
+- name: Run the DATUM Gateway health check once to arm the timer
+  command: systemctl start datum-gateway-healthcheck.service
+  changed_when: false

ansible/roles/datum_gateway/tasks/install.yml

@@ -0,0 +1,74 @@
+---
+- name: Install DATUM Gateway build dependencies
+  apt:
+    name:
+      - cmake
+      - build-essential
+      - git
+      - libjansson-dev
+      - libmicrohttpd-dev
+      - libsodium-dev
+      - libcurl4-openssl-dev
+      # Runtime-only (netcat for health check)
+      - netcat-openbsd
+    state: present
+    update_cache: yes
+
+# ===========================================
+# System User and Directories
+# ===========================================
+- name: Create datum system user
+  user:
+    name: "{{ datum_gateway_user }}"
+    system: yes
+    shell: /usr/sbin/nologin
+    home: "{{ datum_gateway_dir }}"
+    create_home: no
+    comment: "DATUM Gateway"
+
+- name: Create DATUM Gateway directories
+  file:
+    path: "{{ item.path }}"
+    state: directory
+    owner: "{{ item.owner }}"
+    group: "{{ datum_gateway_group }}"
+    mode: "{{ item.mode }}"
+  loop:
+    - { path: "{{ datum_gateway_dir }}",        owner: root,                   mode: "0755" }
+    - { path: "{{ datum_gateway_source_dir }}", owner: root,                   mode: "0755" }
+    - { path: "{{ datum_gateway_config_dir }}", owner: "{{ datum_gateway_user }}", mode: "0750" }
+    - { path: "{{ datum_gateway_log_dir }}",    owner: "{{ datum_gateway_user }}", mode: "0750" }
+
+# ===========================================
+# Build from Source
+# ===========================================
+- name: Clone DATUM Gateway repository at {{ datum_gateway_version }}
+  git:
+    repo: https://github.com/OCEAN-xyz/datum_gateway.git
+    dest: "{{ datum_gateway_source_dir }}"
+    version: "{{ datum_gateway_version }}"
+    force: yes
+  register: git_clone
+
+- name: Configure cmake build
+  command: cmake . -DCMAKE_BUILD_TYPE=Release
+  args:
+    chdir: "{{ datum_gateway_source_dir }}"
+
+- name: Compile datum_gateway
+  command: make -j{{ datum_gateway_build_jobs }}
+  args:
+    chdir: "{{ datum_gateway_source_dir }}"
+
+- name: Install datum_gateway binary
+  copy:
+    src: "{{ datum_gateway_source_dir }}/datum_gateway"
+    dest: "{{ datum_gateway_bin_path }}"
+    remote_src: yes
+    owner: root
+    group: root
+    mode: "0755"
+  notify: Restart datum-gateway
+
+# ===========================================
+# Configuration

ansible/roles/datum_gateway/tasks/main.yml

@@ -0,0 +1,6 @@
+---
+# import_tasks, not include_tasks: static imports stay visible to --list-tasks.
+- ansible.builtin.import_tasks: install.yml
+- ansible.builtin.import_tasks: configure.yml
+- ansible.builtin.import_tasks: service.yml
+- ansible.builtin.import_tasks: healthcheck.yml

ansible/roles/datum_gateway/tasks/service.yml

@@ -0,0 +1,16 @@
+---
+- name: Reload systemd daemon
+  systemd:
+    daemon_reload: yes
+
+- name: Enable and start datum-gateway
+  systemd:
+    name: datum-gateway
+    enabled: yes
+    state: started
+
+# ===========================================
+# Health Check Script + Systemd Timer
+# ===========================================
+# ═════════════════════════════════════════════════════════════════════════
+# DEPRECATED — Uptime Kuma was decommissioned on 2026-09-11.