feat(status): unified Status page + widget across ping/DNS/HTTP monitors

Adds a Kuma-style "is everything up?" surface that aggregates heterogeneous
monitor types via a status-source registry (steward/core/status.py): each
type registers an async source(db) -> [StatusEntry]. Core registers ping/DNS;
the http plugin registers its own from setup() so core never imports plugin
tables. Per entry: current up/down, last-30 heartbeat bar, uptime %
(24h/7d/30d), latest latency + response sparkline, and TLS expiry countdown
(HTTP). New /status page (live htmx refresh) + a status_overview dashboard
widget + nav link. Pure-function unit tests for registry + sparkline.

First deliverable of milestone #68 (task #866).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/status/index.html

@@ -0,0 +1,12 @@
+{% extends "base.html" %}
+{% block title %}Status — Steward{% endblock %}
+{% block content %}
+<div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1.25rem;gap:1rem;flex-wrap:wrap;">
+  <h1 class="page-title" style="margin-bottom:0;">Status</h1>
+  <span style="font-size:0.78rem;color:var(--text-dim);">Live — refreshes every {{ poll_interval }}s</span>
+</div>
+
+<div hx-get="/status/rows" hx-trigger="load, every {{ poll_interval }}s" hx-swap="innerHTML">
+  <div class="empty">Loading status…</div>
+</div>
+{% endblock %}