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/defaults/main.yml

@@ -0,0 +1,39 @@
+---
+# signal-cli-rest-api: the transport Gatus uses to send Signal messages.
+#
+# Gatus does not speak Signal. It POSTs JSON to this service, which holds the
+# actual Signal identity and does the protocol work.
+
+# Pinned by digest for the same reason as Gatus: a tag is mutable.
+# Upstream publishes no versioned tags worth pinning to, so this pins the
+# DIGEST that `latest` resolved to when this was reviewed. `latest` is a moving
+# target; a digest is a content address, and `docker compose pull` either
+# fetches exactly this image or fails.
+signal_api_image_digest: "sha256:2399d449123cdad56c4d859277e3b9127e1a00c4d2ab4601c239882609286cf8"
+signal_api_image: "bbernhard/signal-cli-rest-api@{{ signal_api_image_digest }}"
+
+signal_api_dir: /opt/signal-api
+signal_api_data_dir: "{{ signal_api_dir }}/data"
+
+# MODE matters on this host. Upstream offers normal / native / json-rpc /
+# json-rpc-native. json-rpc keeps a resident JVM daemon and upstream describes it
+# as "increased memory" - this VPS has 464MB total and already runs Gatus and
+# Caddy, so a resident JVM is not affordable. `native` runs a precompiled
+# GraalVM binary per request: no daemon, no resident cost, and alerts are rare
+# enough that paying startup per alert is the right trade.
+signal_api_mode: native
+
+# Port INSIDE the shared docker network. Never published to the host: this API
+# has NO AUTHENTICATION of any kind. Anyone who can reach it can send messages
+# as you and read your Signal.
+signal_api_port: 8080
+
+# Both this and Gatus join this network so Gatus can reach the API by service
+# name. Gatus runs in a container, so the host's loopback is NOT reachable from
+# it - this is why a shared network is required rather than a published port.
+signal_api_network: monitoring
+signal_api_service_name: signal-api
+
+# The uid the upstream image drops to (`setpriv --reuid=1000`). The data
+# directory must be owned by it or signal-cli cannot write the account.
+signal_api_uid: 1000