docs: make the overview system map readable on GitHub

[mirrec]

The problem

The system map in docs/how-it-works/01-overview.md is so wide that GitHub shrinks it until the text is too small to read. To read it, you have to open the full-screen viewer and pan around. The diagram ran left to right through 6–7 columns, arrows going back and forth (portal ↔ Triage, Connector ↔ Backoffice) stretched it further, and arrow labels like "signed webhook (ES256) to Client.url" made the gaps between columns wider still. Rendered locally, it was about 3,100 px wide.

Solution

This PR fixes only the overview diagram. The other wide diagrams (mostly the sequence diagrams in 03, 05 and 07) are wide for a different reason, long text on the arrows, so they'll each get their own PR.

• The diagram now runs top to bottom (flowchart TD), so it grows downwards.
• Each request/webhook pair is one two-way arrow (jobs <--> Triage, conn <--> BO), and the email arrow is written the other way round (rsuser <-.- mail). Without that, the top-to-bottom layout came out upside down, with the citizen at the bottom and Backoffice at the top.
• Each arrow label is 1–3 words now (REST, JWT, RS emails). The endpoints, tokens and signatures moved to a new Connections table under the diagram, one row per arrow.

While writing the table I checked the auth details against the code and filled in two that the old diagram left out:

• OPS API → Triage uses TRIAGE_ZAMMAD_API_TOKEN (ApiController uses TriageZammadEnvironment.client).
• Backoffice → Connector webhooks are checked against X-Hub-Signature with the tenant's webhook secret (app/controllers/connector/backoffice/webhooks_controller.rb).

Rendered locally with mermaid-cli, the diagram is about 1,500 px wide instead of 3,100 px.

Verify: open the file on this branch on GitHub and check that the system map reads without zooming.

Demo

Rendered 01-overview.md on this branch, compare with main.

Checklist

• docs/how-it-works/ is updated, or this PR changes no flow (job, webhook, Triage or Backoffice trigger, issue state, email, scheduled job)