Documents: Add automatic background processing

[helje5]

The document (and note) types now supports storing a plain text representation of a document. This is intended for two things:

• FTS
• OCR
• faster access
  OGo Obj/C stores the document blobs in the filesystem, though the storage is technically pluggable, we might be able to refer out to e.g. a WebDAV server (or other document providers).

Columns:

• text_content
• text_content_type (SMALLINT, 0=plain?, 1=markdown, 2=html, ..., should that be a MIME type?)
• text_content_object_version (the version of the document the content relates to)

Those fields should be filled asynchronously, either using a queue or just by cron using a started. It could do various things:

• OCR PDF's and images, e.g. using Tesseract or MarkItDown
• Transcribe audio attachments, e.g. using Whisper
• Generate document thumbnails (where would we store them, as sub-documents, own column?)
• It could also create the ts_vector as part of the update of text_content, the related column would have to be created