File a topic room's photos, text and links as one card, closed with --- #221

Story

As a family member keeping a hobby or a collection in a topic room, I want to post a few photos, some text and some links and have them filed as one card when I am done, so that the topic reads as a record of what happened, not as one entry per message.

Context

In a topic room the archivist files every message the moment it arrives: a photo becomes a note, a link a bookmark, a long text a note whose to-dos go to the topic's list (stacklets/docs/bot/archivist.py, _on_file and _on_text). Three photos and two lines of text make five entries.

Two things come close and do not fit:

• The scan mode (( opens, ) or fertig closes) collects files only, keeps the session in the bot's memory, and files the result to Paperless as one document, also in a topic room.
• The diary (stacklets/memory/bot/diary.py) groups a room's messages into cards, keeps the family's words next to the model's summary, and takes corrections in each card's thread. It runs on the nightly sweep, in the memories room only.

Proposal

• In a topic room, the open card is every message since the last card: photos, text, captions, links, replies. It is read from the room's timeline, so a restart loses nothing and a refile can replay it.
• --- on its own line closes the card. ), fertig and done close it too, so there is one grammar.
• A card nobody closes is closed after a quiet period, and on the nightly sweep at the latest.
• The card is one note: a summary, the family's words as written, the photos, the links under "to read" (still fetched when posted, since pages disappear), and the to-dos on the topic's list.
• The archivist posts the card in the room; a reply in its thread corrects or adds to it, as for captures (fix(archivist): keep what the family writes about a capture #219).
• The archivist stops filing single messages in topic rooms. It still answers questions there. The documents room keeps the scan mode.

Out of scope: grouping by time without a close (the diary's heuristics), the memories room, the documents room.

Definition of done

• A test: photos, text and a link followed by --- file as one card with all of them, and nothing is filed before the ---
• A test: a card left open is closed by the quiet period, and an empty room files nothing
• A test: the card's to-dos land on the topic's list, linked to the card
• On the demo family: a topic with three cards shows three entries on its topic page
• docs/user-guide.md explains ---