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-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"