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/core/widgets.py

@@ -22,6 +22,16 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "poll": True,
         "params": [],
     },
+    "status_overview": {
+        "key": "status_overview",
+        "label": "Status — Overview",
+        "description": "Unified up/down/pending summary across ping, DNS, and HTTP monitors",
+        "hx_url": "/status/widget",
+        "detail_url": "/status",
+        "plugin": None,
+        "poll": True,
+        "params": [],
+    },
     "uptime_summary": {
         "key": "uptime_summary",
         "label": "Uptime / SLA",