alerting: Signal via signal-cli-rest-api, and faster failure detection

Three changes: detection windows tightened, a Signal transport deployed, and
alerts attached to all 84 non-transport endpoints.

── Failing faster ──────────────────────────────────────────────────────────
The heartbeat is a TICKER, not a deadline: Gatus wakes every interval and asks
"did anything arrive in the last interval", so real detection is 1-2x the
window. And a window can only ever be as tight as the push frequency - which is
why two checks moved rather than just having their numbers changed.

  liveness, cpu, ups, service-health, probes   16m -> 11m
  disk, zfs                       daily/30h -> 6-hourly/7h
  backup store + pull job         daily/30h -> 6-hourly/7h
  DNS records                           24h -> 6h
  backup dump                             30h -> 26h

backup-dump stays slow because the dump genuinely is daily. The store-side check
catches the same fault within 6h by reading the source's dump timestamp out of
the artefact filename, so 26h is a backstop rather than the primary signal.

── Thresholds differ by check type, deliberately ───────────────────────────
`failure-threshold` counts consecutive failures, but "consecutive" is a
different amount of wall-clock time per check: a push endpoint produces one
failure per heartbeat window, a pulled one per interval. The default of 3 would
mean 33 minutes on an 11m heartbeat and over a day on a 7h one.

More importantly the heartbeat window ALREADY encodes the tolerance - an 11m
window on a 5-minute push is exactly "one missed push forgiven" - so stacking a
threshold of 3 triples a tolerance that was already chosen. Hence:

  push/heartbeat endpoints   failure-threshold 1
  pulled, 5m (public)        failure-threshold 3   (= 15 minutes)
  pulled, 6h/24h (dns, domain) failure-threshold 1

── The Signal transport ────────────────────────────────────────────────────
roles/signal_api runs signal-cli-rest-api on the observability host, pinned by
digest, MODE=native.

It publishes NO PORTS. The API has no authentication of any kind - anything that
reaches it can send messages as you and read your Signal. Gatus talks to it over
a shared docker network by service name, which is also WHY the network exists:
Gatus runs in a container, so the host's loopback is unreachable from it and a
port published on 127.0.0.1 would not have worked.

MODE=native and not json-rpc because this VPS has 464MB of RAM and already runs
Gatus and Caddy. The json-rpc modes hold a resident JVM; native runs a binary
per request, and alerts are rare enough that startup cost per alert is the right
trade.

Monitored - Gatus polls /v1/health over the same network path the alerts take,
so it proves the delivery route rather than mere container liveness. Deliberately
NOT backed up: the data directory holds Signal private keys and the recovery
path is to link the device again from the phone.

Neither the signal-api endpoint nor Gatus's self-check carries a Signal alert.
If either is down, Signal is precisely what cannot deliver the alert.

── Four traps hit while linking, all now in the role README ────────────────
  * /v1/qrcodelink is BROKEN in native mode - returns "no data to encode" while
    the binary itself emits a perfectly good URI. Not worked around by switching
    MODE, which would put a JVM in the path of every alert permanently.
  * The data dir must be owned by uid 1000, not root. A root-owned 0700 dir
    cannot be traversed by the container user, so linking silently never
    completes and /v1/accounts returns "Failed to read local accounts list".
  * `docker exec` runs as ROOT while the service runs as uid 1000, so without
    --config the account is written to /root/... on the container's ephemeral
    layer. It reports success and is destroyed on the next recreate.
  * The phone reporting "network error" was IPv6: chat.signal.org resolves to
    dualstack AAAA records first, the container has no IPv6 address, and this
    host's IPv6 path is broken - the same edge that 404'd the Go tarball. Fixed
    with a mounted gai.conf that prefers IPv4.

Verified: provider loads (configuredProviders=[signal]), a test message was
delivered and confirmed received, 84 endpoints carry alerts, 86 UP / 0 DOWN.

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

ansible/roles/signal_api/tasks/main.yml

@@ -0,0 +1,93 @@
+---
+- name: Assert Docker is available
+  ansible.builtin.command: docker --version
+  register: signal_docker_check
+  changed_when: false
+
+# Created explicitly rather than by either compose file, so neither stack has to
+# be deployed before the other and neither owns it.
+- name: Ensure the shared monitoring network exists
+  ansible.builtin.command: "docker network create {{ signal_api_network }}"
+  register: signal_net
+  changed_when: "'already exists' not in signal_net.stderr"
+  failed_when:
+    - signal_net.rc != 0
+    - "'already exists' not in signal_net.stderr"
+
+- name: Create the signal-api directory
+  ansible.builtin.file:
+    path: "{{ signal_api_dir }}"
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+
+# Owned by the container's uid, NOT root.
+#
+# The image drops to uid 1000 (`setpriv --reuid=1000`), and a root-owned 0700
+# directory cannot be traversed by uid 1000 - signal-cli then fails to write the
+# account and linking silently never completes, leaving a 39-byte accounts.json
+# with no accounts and the API returning "Failed to read local accounts list".
+#
+# 0700 on uid 1000 is still private: only that uid and root can read the Signal
+# private keys, which is the property actually wanted.
+- name: Create the signal-api data directory owned by the container user
+  ansible.builtin.file:
+    path: "{{ signal_api_data_dir }}"
+    state: directory
+    owner: "{{ signal_api_uid }}"
+    group: "{{ signal_api_uid }}"
+    mode: "0700"
+
+- name: Write the IPv4-preference resolver config
+  ansible.builtin.template:
+    src: gai.conf.j2
+    dest: "{{ signal_api_dir }}/gai.conf"
+    owner: root
+    group: root
+    mode: "0644"
+
+- name: Write the docker compose file
+  ansible.builtin.template:
+    src: docker-compose.yml.j2
+    dest: "{{ signal_api_dir }}/docker-compose.yml"
+    owner: root
+    group: root
+    mode: "0644"
+
+- name: Pull the pinned signal-api image
+  ansible.builtin.command:
+    cmd: docker compose pull
+    chdir: "{{ signal_api_dir }}"
+  register: signal_pull
+  changed_when: "'Downloaded newer image' in signal_pull.stderr or 'Pull complete' in signal_pull.stderr"
+
+- name: Start signal-api
+  ansible.builtin.command:
+    cmd: docker compose up -d --remove-orphans
+    chdir: "{{ signal_api_dir }}"
+  register: signal_up
+  changed_when: "'Started' in signal_up.stderr or 'Created' in signal_up.stderr or 'Recreated' in signal_up.stderr"
+
+- name: Wait for the API to answer
+  ansible.builtin.command:
+    cmd: "docker exec {{ signal_api_service_name }} curl -fsS http://localhost:{{ signal_api_port }}/v1/health"
+  register: signal_health
+  until: signal_health.rc == 0
+  retries: 12
+  delay: 5
+  changed_when: false
+
+- name: Report whether an account is linked yet
+  ansible.builtin.command:
+    cmd: "docker exec {{ signal_api_service_name }} curl -fsS http://localhost:{{ signal_api_port }}/v1/accounts"
+  register: signal_accounts
+  changed_when: false
+  failed_when: false
+
+- name: Show the linking status
+  ansible.builtin.debug:
+    msg: >-
+      {{ 'Linked account(s): ' ~ signal_accounts.stdout
+         if (signal_accounts.stdout | default('[]') | trim) not in ['[]', '', 'null']
+         else 'NO ACCOUNT LINKED YET - this is a one-time manual step, see the role README.' }}