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>

mix.exs

@@ -23,7 +23,7 @@ defmodule SimpleshopTheme.MixProject do
   def application do
     [
       mod: {SimpleshopTheme.Application, []},
-      extra_applications: [:logger, :runtime_tools]
+      extra_applications: [:logger, :runtime_tools, :os_mon]
     ]
   end
 
@@ -76,6 +76,8 @@ defmodule SimpleshopTheme.MixProject do
       {:ex_money, "~> 5.0"},
       {:ex_money_sql, "~> 1.0"},
       {:stripity_stripe, "~> 3.2"},
+      {:error_tracker, "~> 0.7"},
+      {:logger_json, "~> 7.0", only: :prod},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
       {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false}
     ]