docs(magento): describe the database backend where Magento was memory or Temporal only (#739, #1036)

[gplanchat]

What

The Magento passages that still described two backends now describe three: the database backend that resource/durable selects, Temporal when durable/temporal/dsn is set, and the in-process journal otherwise. Declaring both throws BackendSelectionException.

• Concepts page (EN and FR): the note "only two of the four are reachable" and the paragraph on how work reaches a worker ([Task] Update the pages that still describe Magento as memory or Temporal only #1036).
• Configuration page (EN and FR): the Magento column of the host table, with the resource/durable and db/connection/durable entries and durable:setup ([Task] Magento on a database: change the public promise (picker, backends and configuration pages, UPGRADE) #739).
• run-not-progressing (EN and FR): a section for the database backend, and the pickup time the grid shows there ([Task] Update the pages that still describe Magento as memory or Temporal only #1036).
• README.md and src/DurableModule/README.md: "memory and Temporal only" is gone.
• UPGRADE.md: a "Changed" section at the end of "Unreleased" ([Task] Magento on a database: change the public promise (picker, backends and configuration pages, UPGRADE) #739). The module README now points to the database backend where it said Magento would not get a journal of its own.

#842 is already satisfied on main (the re-audit comment on the ticket lists each box), so I changed nothing for it.

Checked

• Facts read in RuntimeFactory::backend(), JournalConnectionResolver, MagentoWorkflowRunCatalog (picked_up_at fills waitingForWorkerSince) and HealthCommand.
• hugo --minify builds. git grep "only two of the four" returns nothing in EN and FR.
• Not run: the picker over HTTP (not changed), anything needing a Magento install.

Not in this pull request

• The landing-page picker (ALLOWED.magento, WHYNOT.magento, FWNOTE.magento in index.html and index.fr.html): [Task] Magento on a database: change the public promise (picker, backends and configuration pages, UPGRADE) #739 leaves the key of the new SQL row to you. Which key?
• Stale docblocks in RuntimeFactory ("Magento reaches no other persistent journal"), outside the documentation.
• durable:health still prints the "No Temporal DSN" message on the database backend; the pages say that durable:health only checks that the journal database answers, and that the grid shows a run that waits for a worker.

Refs #739, #1036