feat(sentinèlla): p2p topology with DNS peer discovery

- Replace central sentinel with watcher: each node polls peers discovered
  via a single DNS name with multiple A records (e.g. peers.sentinella.com)
- Auto-detect own IPs via hostname -I; SELF env var available as optional
  override for NAT/floating-IP setups
- Fix Basic Auth bug in router.sh: compare tok against AUTH_TOKENS instead
  of unset $USER/$PASS
- Rename sentinel binary to watcher; drop unused shellplot dep
- Add inetutils to watcher runtime deps for hostname -I
- Update NixOS module: replace sentinel options with watcher p2p options
  (peersDns, self, peersPort, peersScheme, pollingIntervalSec)
- Add sentinèlla test suite: probe-status-empty, probe-disk, watcher-state-file

test/package/sentinèlla/test/probe-disk.sh

@@ -0,0 +1,35 @@
+#!/bin/dash
+# Test: probe GET /disk returns JSON with at least one volume entry
+
+log notice "test case: ${WHITE}probe GET /disk returns volume data"
+
+PORT=15989
+export PORT URLS="" VOLUMES="/"
+
+probe &
+probe_pid=$!
+trap 'kill $probe_pid 2>/dev/null; exit' EXIT INT HUP
+
+sleep 2
+
+response=$(curl -sS --max-time 5 "http://127.0.0.1:${PORT}/disk")
+log info "response: $WHITE$response"
+
+count=$(printf '%s' "$response" | jq -r '.volumes | length')
+log info "volume count: $WHITE$count"
+
+if [ "$count" -lt 1 ]; then
+  log error "expected at least 1 volume, got $count"
+  exit 1
+fi
+log info "PASS: at least one volume returned"
+
+# each entry must have a mount field
+mount=$(printf '%s' "$response" | jq -r '.volumes[0].mount')
+if [ -z "$mount" ] || [ "$mount" = "null" ]; then
+  log error "volumes[0].mount is missing or null"
+  exit 1
+fi
+log info "PASS: volumes[0].mount = $mount"
+
+log notice "test passed"

test/package/sentinèlla/test/probe-status-empty.sh

@@ -0,0 +1,27 @@
+#!/bin/dash
+# Test: probe responds on GET /status with valid JSON when URLS is empty
+
+log notice "test case: ${WHITE}probe GET /status returns JSON with empty checks"
+
+# start probe on a free port
+PORT=15988
+export PORT URLS="" VOLUMES="/"
+
+probe &
+probe_pid=$!
+trap 'kill $probe_pid 2>/dev/null; exit' EXIT INT HUP
+
+# wait for probe to be ready
+sleep 2
+
+response=$(curl -sS --max-time 5 "http://127.0.0.1:${PORT}/status")
+log info "response: $WHITE$response"
+
+# must be valid JSON with summary.total == 0
+total=$(printf '%s' "$response" | jq -r '.summary.total')
+assert_eq "summary.total is 0 when URLS empty" "$total" "0"
+
+ok=$(printf '%s' "$response" | jq -r '.summary.ok')
+assert_eq "summary.ok is 0 when URLS empty" "$ok" "0"
+
+log notice "test passed"

test/package/sentinèlla/test/watcher-state-file.sh

@@ -0,0 +1,75 @@
+#!/bin/dash
+# Test: watcher writes a state file after polling a peer
+#
+# Setup:
+#   - Start a probe on 127.0.0.1:15990
+#   - Stub getent to resolve peers.test -> 127.0.0.1 (the probe) and 10.0.0.1 (fake peer)
+#   - Stub hostname to return 10.0.0.1 as the local IP so 10.0.0.1 is excluded
+#     and 127.0.0.1 (the real probe) is kept as a peer
+#   - Assert a state file appears in STATE_DIR within 15s
+
+log notice "test case: ${WHITE}watcher writes state file after first successful poll"
+
+PORT=15990
+export PORT URLS="" VOLUMES="/"
+
+probe &
+probe_pid=$!
+trap 'kill "$probe_pid" 2>/dev/null; kill "$watcher_pid" 2>/dev/null; rm -rf "$stub_dir" "$state_dir"' EXIT INT HUP
+
+sleep 2
+
+# Create stubs directory
+stub_dir=$(mktemp -d)
+
+# Stub getent: returns two IPs for peers.test
+cat >"${stub_dir}/getent" <<'EOF'
+#!/bin/sh
+if [ "$1" = "hosts" ] && [ "$2" = "peers.test" ]; then
+  printf '127.0.0.1 peers.test\n'
+  printf '10.0.0.1 peers.test\n'
+else
+  /usr/bin/getent "$@"
+fi
+EOF
+chmod +x "${stub_dir}/getent"
+
+# Stub hostname: -I returns 10.0.0.1 so watcher excludes it and keeps 127.0.0.1
+cat >"${stub_dir}/hostname" <<'EOF'
+#!/bin/sh
+case "$1" in
+  -I) printf '10.0.0.1\n' ;;
+  *)  /bin/hostname "$@" ;;
+esac
+EOF
+chmod +x "${stub_dir}/hostname"
+
+state_dir=$(mktemp -d)
+
+export PEERS_DNS="peers.test"
+export PEERS_PORT="$PORT"
+export PEERS_SCHEME="http"
+export TG_TOKEN="test-token"
+export TG_CHAT_ID="test-chat"
+export STATE_DIR="$state_dir"
+export POLLING_INTERVAL_SEC="1"
+export SPAM="0"
+unset SELF  # ensure auto-detection is used
+
+PATH="${stub_dir}:${PATH}" watcher &
+watcher_pid=$!
+
+log info "waiting for state file in $state_dir ..."
+peer_url="http://127.0.0.1:${PORT}"
+state_file="${state_dir}/$(printf '%s' "$peer_url" | cksum | awk '{print $1}').state"
+wait_for_file "$state_file" 15
+
+state=$(cat "$state_file")
+log info "state file content: $WHITE$state"
+
+case "$state" in
+  up:*|down:*) log info "PASS: state file has expected format" ;;
+  *) log error "unexpected state file content: $state"; exit 1 ;;
+esac
+
+log notice "test passed"