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>

enodia_sentinel/notify/backends.py

@@ -0,0 +1,100 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""Phone-push backends. Each separates a pure ``build()`` (testable, no network)
+from ``send()`` (does the request), so request construction is unit-tested
+without ever hitting the wire."""
+from __future__ import annotations
+
+import json
+import urllib.parse
+import urllib.request
+
+from ..alert import Severity
+from ..config import Config
+
+_TIMEOUT = 6
+
+# Severity -> per-service priority/tags.
+_NTFY_PRIORITY = {Severity.MEDIUM: "3", Severity.HIGH: "4", Severity.CRITICAL: "5"}
+_NTFY_TAGS = {Severity.MEDIUM: "mag", Severity.HIGH: "warning",
+              Severity.CRITICAL: "rotating_light"}
+_PUSHOVER_PRIORITY = {Severity.MEDIUM: "-1", Severity.HIGH: "0", Severity.CRITICAL: "1"}
+
+
+def _send(req: urllib.request.Request) -> bool:
+    with urllib.request.urlopen(req, timeout=_TIMEOUT) as resp:
+        return 200 <= resp.status < 300
+
+
+class Ntfy:
+    name = "ntfy"
+
+    @staticmethod
+    def enabled(cfg: Config) -> bool:
+        return bool(cfg.notify_ntfy_url and cfg.notify_ntfy_topic)
+
+    @staticmethod
+    def build(cfg: Config, notif) -> urllib.request.Request:
+        url = f"{cfg.notify_ntfy_url.rstrip('/')}/{cfg.notify_ntfy_topic}"
+        headers = {
+            "Title": notif.title(),
+            "Priority": _NTFY_PRIORITY.get(notif.severity, "4"),
+            "Tags": _NTFY_TAGS.get(notif.severity, "warning"),
+        }
+        if cfg.notify_ntfy_token:
+            headers["Authorization"] = f"Bearer {cfg.notify_ntfy_token}"
+        return urllib.request.Request(
+            url, data=notif.message().encode("utf-8"),
+            headers=headers, method="POST")
+
+    @classmethod
+    def send(cls, cfg: Config, notif) -> bool:
+        return _send(cls.build(cfg, notif))
+
+
+class Pushover:
+    name = "pushover"
+
+    @staticmethod
+    def enabled(cfg: Config) -> bool:
+        return bool(cfg.notify_pushover_token and cfg.notify_pushover_user)
+
+    @staticmethod
+    def build(cfg: Config, notif) -> urllib.request.Request:
+        form = urllib.parse.urlencode({
+            "token": cfg.notify_pushover_token,
+            "user": cfg.notify_pushover_user,
+            "title": notif.title(),
+            "message": notif.message(),
+            "priority": _PUSHOVER_PRIORITY.get(notif.severity, "0"),
+        }).encode("utf-8")
+        return urllib.request.Request(
+            "https://api.pushover.net/1/messages.json", data=form,
+            headers={"Content-Type": "application/x-www-form-urlencoded"},
+            method="POST")
+
+    @classmethod
+    def send(cls, cfg: Config, notif) -> bool:
+        return _send(cls.build(cfg, notif))
+
+
+class Webhook:
+    name = "webhook"
+
+    @staticmethod
+    def enabled(cfg: Config) -> bool:
+        return bool(cfg.notify_webhook_url)
+
+    @staticmethod
+    def build(cfg: Config, notif) -> urllib.request.Request:
+        body = json.dumps({
+            "title": notif.title(),
+            "message": notif.message(),
+            **notif.to_dict(),
+        }).encode("utf-8")
+        return urllib.request.Request(
+            cfg.notify_webhook_url, data=body,
+            headers={"Content-Type": "application/json"}, method="POST")
+
+    @classmethod
+    def send(cls, cfg: Config, notif) -> bool:
+        return _send(cls.build(cfg, notif))