Dashboard: demote Worker Status to a health summary, hide infra internals

[alexeygrigorev]

Part of UX epic #67. From the operator UX audit (finding 1, HIGH).

Problem

The most prominent panel on the landing page is a Worker Status table full of systemd unit names, process_sqs_worker commands, PIDs, and red "Missing" badges. It reads like a server console and is the first thing an operator sees. (templates/mailing/dashboard.html:110-160, mailing/services/worker_status.py:57-93)

Ask

Default to a compact "Background processing" health line (healthy / degraded / N waiting) and keep the operationally-useful backlog counts. Move service names, commands, PIDs, and started_at into a <details> diagnostics disclosure. Drop the command column from the default view.

Acceptance

• Dashboard default view shows a one-line processing health summary + backlog counts, no systemd/command/PID text.
• Full diagnostics available behind a disclosure.
• Django template test; lint passes.