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/briefs.py

@@ -1,8 +1,8 @@
 from __future__ import annotations
 
 import sqlite3
-from datetime import date
 
+from .localtime import local_today
 from .paywall import is_paywalled
 
 
@@ -13,7 +13,7 @@ def build_daily_brief(
     replace: bool = False,
     window_days: int = 3,
 ) -> int:
-    target_date = brief_date or date.today().isoformat()
+    target_date = brief_date or local_today()
 
     # Compose the selection first so we can tell whether anything actually
     # changed. A calm daily brief shouldn't repeatedly hand the reader a locked