gatus: deploy on prd-monitoring, behind Caddy basic auth

First step of replacing Uptime Kuma and ntfy. Gatus runs on the new
observability VPS, fronted by Caddy at status.contrapeso.xyz.

Deployed as the upstream container image, not built from source. The role did
build from source first - their Dockerfile is a bare `CGO_ENABLED=0 go build`,
the Vue dashboard is compiled in via `//go:embed static` in web/static.go, and
CGO can stay off because the sqlite driver is pure-Go modernc.org/sqlite - but
that produces a binary upstream never ran, and it meant compiling the AWS SDK
and gRPC on the smallest box in the estate. That load was heavy enough that
unrelated Ansible tasks timed out while it ran. The cost of the container is a
daemon on the machine whose job is to notice when everything else breaks; that
trade is made deliberately and is written down in the role README.

Pinned by DIGEST, not tag. A tag is mutable - v5.36.0 can be repushed - so
pinning it alone is a weaker promise than it looks:

    gatus_image: "ghcr.io/twin/gatus@sha256:c5f210d0..."

`docker compose pull` now either fetches exactly the reviewed image or fails.
gatus_version is kept beside it only so a human can read the release; the two
move together.

The image is FROM scratch, so it has no /etc/passwd and its default user is
root. The container runs as 10001:10001 with the host data dir owned to match,
plus read_only, cap_drop ALL, and no-new-privileges. NET_RAW is added back only
when gatus_allow_icmp, so the capability for icmp:// checks is a visible grant
rather than something inherited from running as root.

Config is a DIRECTORY, not a file. Gatus merges every *.yaml under
GATUS_CONFIG_PATH - maps deep-merge, lists append - so the role owns
00-base.yaml (web, storage, ui, alerting, security) and each service will drop
its own file into endpoints/, the same shape as caddy_site. A primitive defined
twice is ambiguous and upstream refuses it, so anything that is not a list lives
in the base file and nowhere else.

Two bugs the deploy caught:

  * Gatus panics on a config with no endpoints ("configuration should contain at
    least one endpoint or suite"), so "install now, add endpoints later" is not
    a valid state. The role ships endpoints/00-self.yaml checking its own
    /health. Less circular than it looks: it proves the directory merged, the
    listener serves, and storage accepted a write.

  * web.address was carried over from the systemd design as 127.0.0.1. Inside a
    container that is the CONTAINER's loopback, which docker-proxy cannot reach
    - gatus came up healthy, self-check passing, while every connection to the
    published port was refused. It now always binds 0.0.0.0 inside the
    container; the isolation comes from publishing to 127.0.0.1 on the host.

Auth is done at the edge, NOT with Gatus's own security.basic. Reading
api/api.go, that middleware protects exactly four routes - the statuses
endpoints. Everything else is registered on the unprotected router, including
/api/v1/config, every badge, and /api/v1/endpoints/:key/uptimes/:duration and
.../response-times/:duration/history, which return real data to anyone who can
guess a key ("<group>_<name>"). Verified against the live instance: all seven
routes returned 200 unauthenticated, and /uptimes/24h returned "1.000000".

So the vhost uses caddy_site_body with a path carve-out rather than
caddy_site_basic_auth, which has no way to exempt a path. The external-endpoint
push API must NOT sit behind basic auth: it authenticates with
`Authorization: Bearer <token>`, and basic auth wants the same header. It is not
unauthenticated - the handler 401s on a missing prefix, an empty token, or a
token that does not match that endpoint's own.

Verified end to end. All seven previously-open routes now 401. The push path
distinguishes cleanly: POST with no auth gets Gatus's own "invalid Authorization
header" with NO WWW-Authenticate; POST with a bogus Bearer gets 404 (key looked
up, no external endpoints yet); GET on the same path gets Caddy's 401 with
WWW-Authenticate: Basic, so the exemption is scoped to POST alone. The
self-check still passes because it polls localhost inside the container and
never traverses Caddy.

The host itself was rebuilt from scratch: 01 (ok=9 changed=8), 02 (ok=12
changed=6), 910_docker (--limit, since that playbook still wrongly claims all of
`managed` needs Docker), caddy (ok=13 changed=8), gatus (ok=18 changed=2).

Not done here: gatus_alerting is still {} - valid, and every condition is
evaluated and recorded, there is just nowhere to shout until a provider is
chosen to replace ntfy.

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

ansible/roles/gatus/defaults/main.yml

@@ -0,0 +1,83 @@
+---
+# Gatus, deployed the way upstream distributes it: the container image.
+#
+# Upstream publishes NO binary release assets - the image is the only artefact
+# they ship, and therefore the only artefact they test. Building from source is
+# possible (`go build` alone is enough; the Vue dashboard is compiled in via
+# `//go:embed static`, and CGO_ENABLED=0 works because the sqlite driver is
+# pure-Go modernc.org/sqlite) but it produces a binary upstream never ran, and
+# it means a full compile of the AWS SDK and gRPC on the smallest box in the
+# estate.
+
+# ── Image ────────────────────────────────────────────────────────────────────
+gatus_version: "v5.36.0"
+# Pinned by DIGEST, not by tag. A tag is mutable - `v5.36.0` can be repushed -
+# so pinning the tag alone is a weaker promise than it looks. The digest is the
+# content address: if it resolves, it is byte-for-byte the image reviewed here.
+# Both must be updated together; the tag is kept only so humans can read it.
+gatus_image_digest: "sha256:c5f210d095fa78e6efaa20ffeb14803f2ba4f10615e16a6d12087697149617f0"
+gatus_image: "ghcr.io/twin/gatus@{{ gatus_image_digest }}"
+
+# ── Paths (host side) ────────────────────────────────────────────────────────
+gatus_dir: /opt/gatus
+gatus_config_dir: "{{ gatus_dir }}/config"
+gatus_data_dir: "{{ gatus_dir }}/data"
+
+# Gatus merges every *.yaml under GATUS_CONFIG_PATH and its subdirectories:
+# maps deep-merge, lists append. That is why this role ships a config DIRECTORY
+# rather than one file - each service contributes its own endpoint file, the
+# same way each service contributes a vhost through `caddy_site`.
+#
+# Primitives must be defined exactly once across all files or the merge is
+# ambiguous, so everything that is not a list lives in the base file and
+# nowhere else.
+gatus_base_config_file: "00-base.yaml"
+gatus_endpoints_dir: "{{ gatus_config_dir }}/endpoints"
+
+# ── Identity ─────────────────────────────────────────────────────────────────
+# The image is FROM scratch, so it has no /etc/passwd and no user to drop to by
+# name. Run it by numeric uid/gid instead, and own the data volume to match.
+gatus_uid: 10001
+gatus_gid: 10001
+
+# ── Web ──────────────────────────────────────────────────────────────────────
+gatus_port: 8080
+# HOST-side address the container's port is published on. Gatus itself always
+# binds 0.0.0.0 inside the container - see the note in config.yaml.j2. Never
+# publish this on 0.0.0.0: the external-endpoint push API shares the dashboard's
+# listener, and Caddy is what should be in front of both.
+gatus_bind_address: "127.0.0.1"
+gatus_ui_title: "Status"
+gatus_ui_header: "Status"
+
+# ── Storage ──────────────────────────────────────────────────────────────────
+# sqlite, not memory: history has to survive a restart, or the dashboard lies
+# about uptime after every deploy. Path is INSIDE the container.
+gatus_storage_type: sqlite
+gatus_storage_path: "/data/gatus.db"
+gatus_storage_caching: true
+
+# ── Alerting ─────────────────────────────────────────────────────────────────
+# Pass-through: rendered verbatim under `alerting:`, so any provider Gatus
+# supports works without touching this role. Empty means "check and record,
+# alert nowhere" - valid, and the default until a provider is chosen.
+gatus_alerting: {}
+gatus_default_alerts: []
+
+# ── Security ─────────────────────────────────────────────────────────────────
+#   gatus_basic_auth: {username: admin, password-bcrypt-base64: "..."}
+gatus_basic_auth: {}
+
+gatus_maintenance: {}
+gatus_log_level: INFO
+
+# ── Self-check ───────────────────────────────────────────────────────────────
+# Gatus panics on a config with no endpoints, so the role always ships one.
+# Turning this off is only safe once another file in endpoints/ provides one.
+gatus_self_check: true
+
+# ── ICMP ─────────────────────────────────────────────────────────────────────
+# Gatus supports icmp:// endpoints. Raw ICMP needs CAP_NET_RAW, which the
+# container would get free only if it ran as root; it does not. Set false if
+# you never use icmp:// checks and want the capability dropped entirely.
+gatus_allow_icmp: true