add observability: LiveDashboard in prod, error tracking, JSON logging

- Move LiveDashboard to /admin/dashboard behind session auth (all envs)
- Add ErrorTracker at /admin/errors for auto-captured exceptions
- Add Oban job and LiveView metrics to telemetry module
- Add logger_json for structured JSON logs in production
- Enable os_mon for CPU/disk/memory in LiveDashboard OS Data tab
- Extend logger metadata with oban_worker and oban_queue fields

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

priv/repo/migrations/20260208165931_add_error_tracker.exs

@@ -0,0 +1,6 @@
+defmodule SimpleshopTheme.Repo.Migrations.AddErrorTracker do
+  use Ecto.Migration
+
+  def up, do: ErrorTracker.Migration.up(version: 5)
+  def down, do: ErrorTracker.Migration.down(version: 1)
+end