Dashboard: content + source-health; per-viewer local dates

* Date fix: introduce GOODNEWS_TZ (goodnews/localtime.py) so the brief's "today"
  rolls over in a pinned zone (Eastern) instead of UTC — robust to host-clock
  resets. The home page now formats the brief's date in each VISITOR's local
  timezone (from its UTC freshness stamp), so nobody ever sees "tomorrow."

* Admin "Content served": articles live, fresh (7d), ingested (24h), summaries,
  active sources, today's brief size — queries.content_stats().

* Admin "Source health": per active source, the failure streak, last error,
  accepted contribution, and computed next-poll time (so backoff / "resting
  until" is visible), via queries.source_health() reusing the feeds backoff
  math. Failing sources sort to the top; times render in the viewer's zone.

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

goodnews/localtime.py

@@ -0,0 +1,32 @@
+"""The site's canonical local day.
+
+There is one daily brief, so "today" must resolve to a single timezone — not the
+host clock (which is UTC on the server and was silently reset on a rebuild).
+GOODNEWS_TZ pins it explicitly (default UTC) so a rebuild can't reintroduce the
+"brief shows tomorrow's date in the evening" bug. This governs when the brief
+rolls over and the canonical date on the server-rendered /today page; each
+visitor's *displayed* date is formatted in their own browser timezone.
+"""
+
+from __future__ import annotations
+
+import os
+from datetime import datetime
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+
+
+def site_tz() -> ZoneInfo:
+    name = os.environ.get("GOODNEWS_TZ", "UTC")
+    try:
+        return ZoneInfo(name)
+    except (ZoneInfoNotFoundError, ValueError):
+        return ZoneInfo("UTC")
+
+
+def local_now() -> datetime:
+    return datetime.now(site_tz())
+
+
+def local_today() -> str:
+    """The site's current calendar date (ISO yyyy-mm-dd) in GOODNEWS_TZ."""
+    return local_now().date().isoformat()