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>

config/prod.exs

@@ -17,5 +17,8 @@ config :swoosh, local: false
 # Do not print debug messages in production
 config :logger, level: :info
 
+# Structured JSON logs for production (machine-parseable by fly logs, journalctl, Loki, etc.)
+config :logger, :default_handler, formatter: {LoggerJSON.Formatters.Basic, []}
+
 # Runtime production configuration, including reading
 # of environment variables, is done on config/runtime.exs.