Integrate the remaining modernization work on dev and cut a release candidate

## Goal

Make `dev` the single tested integration point for the modernized stack before promoting it to `master`.

## Context

- PR #171 (PostgreSQL dual writes and metrics) is merged into `dev`.
- PR #172 (React/MUI/tooling modernization and UI fixes) is green but still targets `master`.
- PR #140 (preserve room solves by hiding expired rooms) is based on an old `master` and currently conflicts.
- `dev` is six commits ahead of `master`.

## Acceptance criteria

- [ ] Rebase or retarget #172 onto the latest `dev`, resolve overlaps with the PostgreSQL/shared-module work, and keep all checks green.
- [ ] Triage #140 against #160: either defer it into the persistence sequence or rebase/split it if its scope is intentionally included; it must not silently block the base modernization release.
- [ ] Complete #177 so the integrated `dev` merge commit is checked and the promotion process is documented.
- [ ] Run lint, unit tests, build, Cypress, and Prisma schema/migration checks on the integrated release candidate.
- [ ] Open a reviewed `dev → master` promotion PR with release notes and known rollout risks.
- [ ] Close or supersede obsolete open branches/PRs only after their unique changes are accounted for.

## Non-goal

Do not bundle new product features into the release candidate beyond work explicitly accepted during release triage.

Beginne damit, die PRs #171, #172, #140, #160 und #177 zusammen mit den aktuellen dev- und master-Historien zu prüfen. Gleiche die Arbeiten an Modernisierung, Persistenz und room-solve ab, ohne nicht triagierte Features hinzuzufügen, und führe anschließend lint, Unit-Tests, den Build, Cypress sowie die Prisma-Schema-/Migrationsprüfungen aus. Erledigt ist die Aufgabe, wenn ein geprüftes dev-to-master-Promotion-PR vorliegt, das Release Notes und bekannte Rollout-Risiken enthält.

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