fix(requests): consistent processing state and pick-up of due retries on the showcase

[Fluory]

Goal

A visitor always sees the same, understandable state in list and detail, and a due retry runs without waiting for the daily cron. Context: showcase review 2026-09-27 rated the live demo 4/10 because a request stayed "In Verarbeitung"; root cause measured in #67: the Gemini API free tier answers about one call in three with 503 UNAVAILABLE (high demand), and on Vercel Hobby a job retry waits for a user action or the daily cron. Epic #19.

Acceptance criteria

• The detail page shows the same processing state as the list: human-readable last error, attempts, next retry – instead of only "Die Dokumente werden gerade ausgewertet"
• With JOB_DRAIN_INLINE=true, opening the request list or a request detail runs one bounded drain after the response (after()), so due retries are picked up by normal use
• The drain on page view is throttled (at most once per 30 s per instance) and never delays the page
• Without JOB_DRAIN_INLINE nothing changes (local/production worker path)

Not part of this task

• New queue settings, a manual "retry now" for jobs that are not yet due

Test plan

Criterion	Check
state shown in detail	integration test with a request in retry state
drain on view + throttle	unit test of the throttle; integration test that a due job is processed after a page view

Security/Privacy affected?

No – error texts stay human-readable codes without document content.

[Fluory]

Claimed by @Fluory on branch claude/fix-processing-state-70 (stacked on #72).