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>

plugins/http/__init__.py

@@ -12,6 +12,10 @@ def setup(app: "Quart") -> None:
     global _app
     _app = app
     from .models import HttpMonitor, HttpResult  # noqa: F401
+    # Contribute HTTP monitors to the core unified Status page.
+    from steward.core.status import register_status_source
+    from .routes import http_status_source
+    register_status_source(http_status_source)
 
 
 def get_scheduled_tasks() -> list: