Support PDF attachments with a first-page preview

[lylo]

PDFs couldn't be attached to posts at all. Worksheets, papers and zines are exactly the sort of thing people want to hang off a post, and Lexxy already supports them end to end; the block was ours.

What changed

• application/pdf joins UploadLimits::CONTENT_TYPES at 10MB, free tier, counting against the free quota like images. That one constant gates the editor, API, Micropub and posting by email.
• A GeneratePdfPreviewJob renders the first page to a PNG once, triggered when the blob is attached, so all four ingress routes are covered. Requires pdftoppm (poppler), already on the server and now installed in CI; without it the job no-ops and the chip renders instead.
• The blob partial renders a PDF as a first-page thumbnail with the caption, a Download link and file size beneath, all class-hooked so custom CSS can restyle or hide each part (documented in the help guide, unsynced). The partial previously emitted an empty <figure> for any non-media type, which also fed digest emails, both RSS feeds and exports.
• Exports bundle storage-hosted links as well as images, so the PDF file lands in the zip. The figcaption Markdown converter now converts its children rather than flattening to text, so the Download link survives in Markdown exports.
• The lightbox skips PDF thumbnails, and figure extension classes are downcased to match the editor (prod custom CSS audited, no uppercase selectors).

Behaviour changes

• New posts can attach PDFs; existing content renders byte-identically apart from the downcased extension class.
• The Download link opens the file in a new tab in production (cross-origin ignores the download attribute); a true attachment-disposition download is deliberately left for later.
• PDFs cannot join image galleries; that's upstream in Lexxy (Dragging a PDF onto another PDF deletes it basecamp/lexxy#1235, #1236, #1237).