Add read-only web dashboard and phone push notifications

Both zero-dependency (stdlib http.server + urllib), consistent with the
project's no-dependency-tree stance for a security daemon.

Web dashboard (enodia_sentinel/web.py + static/dashboard.html):
- read-only JSON API over the log dir: /api/status, /api/alerts,
  /api/alerts/<id>, /api/events
- bearer-token auth (constant-time; header or ?token=), required on non-
  loopback binds, auto-generated + persisted (0600) when unset
- binds the host's Tailscale IP by default (auto-detected), reachable from the
  tailnet but not the LAN/internet
- self-contained dark SPA: severity cards, live alert list, full snapshot
  viewer; 10s auto-refresh
- path-traversal-safe alert lookup; `enodia-sentinel web` subcommand; daemon
  now writes a pidfile so the dashboard can show live status
- hardened enodia-sentinel-web.service (read-only, no caps)

Phone push (enodia_sentinel/notify/):
- pluggable backends — ntfy, Pushover, generic webhook — each separating a pure
  build() (unit-tested, no network) from send()
- a backend turns on when its config keys are set; pushes gated by
  notify_min_severity; severity → per-service priority/tags
- fired from snapshot.capture on worker threads, errors swallowed
- desktop notify-send retained

Tests: +16 (9 web incl. a real-server 401/200 auth test, 7 notify request-build
cases). 55/55 pass. Live end-to-end verified: daemon → alert → API → page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

systemd/enodia-sentinel-web.service

@@ -0,0 +1,33 @@
+[Unit]
+Description=Enodia Sentinel - read-only web dashboard
+After=network-online.target enodia-sentinel.service
+Wants=network-online.target
+Documentation=https://github.com/Enodia/enodia-sentinel
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/env enodia-sentinel web
+Restart=on-failure
+RestartSec=5
+
+# The dashboard only READS the log dir and binds a port. No special privileges:
+# lock it down hard. It does not need root, but reads root-owned snapshots, so
+# it runs as root with an otherwise minimal surface and no capabilities.
+ProtectSystem=strict
+# Writable only so it can persist an auto-generated token; set web_token in the
+# config to keep this purely read-only.
+ReadWritePaths=/var/log/enodia-sentinel
+ProtectHome=yes
+NoNewPrivileges=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+MemoryDenyWriteExecute=yes
+LockPersonality=yes
+RestrictNamespaces=yes
+CapabilityBoundingSet=
+AmbientCapabilities=
+
+[Install]
+WantedBy=multi-user.target