Deleting the last subpage does not convert the parent back from {Titel}/Readme.md to {Titel}.md — orphaned folder page renders broken

[tilllt]

Describe the bug

About this report: This analysis was produced automatically by Hermes (an autonomous AI agent) during a systematic debugging session against a live Nextcloud instance. All reproduction steps were executed against the real API/WebDAV; versions are from the instance's own status endpoints.

When the last subpage of a page is deleted, the parent page is not converted back from its subfolder form ({Titel}/Readme.md) to a regular page file ({Titel}.md). The page stays as a folder with a Readme.md but no remaining subpages. Such an "orphaned folder page" is then not rendered correctly in the Collectives UI: the page's content is not shown as a normal page — the user sees raw/HTML garbage below the rendered footer (or the page appears broken / not visible as a page at all).

Additionally, the parent's subpageOrder still references the deleted child's page ID (a stale ID — related to #2588, but the folder-conversion aspect is not covered there).

To Reproduce

Steps to reproduce the behavior (via the Collectives REST API v1.0 + WebDAV):

1. Create a page P (parent page), e.g. POST /collectives/1/pages with title P.
   • The page is stored as a regular file: P.md (WebDAV: .../.Kollektive/<collective>/P.md), API reports fileName: "P.md".
2. Create a subpage C of P (parentId = P's ID).
   • Collectives auto-converts P to a folder: WebDAV now shows P/Readme.md, API reports fileName: "Readme.md", filePath: "P", subpageOrder: [C-id].
3. Delete C (move to trash) and permanently purge it.
   • Expected: P converts back to a regular file P.md.
   • Actual: P remains a folder P/Readme.md with an empty subpageOrder — the reverse conversion never happens.
4. Open page P in the Collectives UI.
   • Expected: page content rendered as a normal page.
   • Actual: the page is not rendered as a regular page — the raw Markdown/HTML content shows up as broken "HTML garbage" below the footer; the page appears broken or is not shown in the page list at all.

Expected behavior

• When the last subpage is deleted, the parent should be converted back from {Titel}/Readme.md (folder form) to {Titel}.md (regular page file), mirroring the automatic Page.md → Page/Readme.md conversion that happens when the first subpage is created.
• The parent's subpageOrder should be cleaned up when a child page is deleted (see Subpage ID not deleted when page is deleted #2588).
• An "orphaned folder page" (folder + Readme.md, zero subpages) should never be presented to the UI in a broken state.

Screenshots

The broken rendering: a page stored as My-Page/Readme.md (folder, no subpages) showed its content partially rendered, followed by a block of raw HTML/XML below the footer instead of the Markdown being rendered as a normal page. After manually restoring the file to My-Page.md via WebDAV MOVE, the page rendered correctly.

Server details:

• Collectives app version: 4.4.1
• Nextcloud version: 33.0.5
• PHP Version: managed (AIO-like environment)
• Database: managed

Client details:

• OS: any (server-side rendering issue)
• Browser: any

Logs

No server exceptions were observed in the Nextcloud log during reproduction. The issue is purely about page storage layout + UI rendering.