[Oceandek]

Closes #95 (spec #91).

What's new

• Message menu (⋮ on every message in a writable chat): Reply for everyone, Edit on your own messages, Delete on your own, and on every message for Teachers and the Superuser (moderation). Students get 403 on others' messages.
• Edit: a dialog with the text; the message then shows "(edited)". Only the author can edit, never staff.
• Delete: after a confirmation, the text and the files are removed (rows and files on the volume); the row stays as "This message was deleted.". A deleted message is never Unread, can't be edited, replied to or deleted again (409/422).
• Reply with a quote: "Replying to : " above the send box (× to stop); the reply shows the quoted author and first line (or the first file's name) with a red line, and clicking it scrolls to the original. A quote of a deleted message says "This message was deleted.". Replies only within the same chat (422).
• Live: every change gets a new number from the chat_revision sequence; an open chat now asks for everything changed since the last number it saw, so edits and deletions appear within ~2 s. Changing or deleting a message also renumbers the replies quoting it, so their quotes update too. ADR 0008 updated.
• Migration 0017: reply_to_id (SET NULL), edited_at, deleted_at, revision (existing messages numbered in order; checked locally by downgrading to 0016 with messages and upgrading). CONTEXT.md: Reply, Deleted message; changelog row.

API

PUT /api/chat/messages/{id} (author), DELETE /api/chat/messages/{id} (author or staff), reply_to_id form field on send. GET .../chat/messages takes since (a revision) instead of after (a message id); messages get revision, edited, deleted, reply_to {id, author_name, first_line, deleted}, can_edit, can_delete.

Also fixed

An open chat moved its "last seen" mark to your own just-sent message, so a message someone else sent a moment earlier could be skipped until the page was reloaded. Now only what a poll fetched moves the mark.

Tests

• Backend 352 passed; new test_chat_edit_delete.py (edit own + live via since, nobody edits others', delete own incl. files and Unread, staff delete any / Student only own, outsiders and archived chats, reply quote, quote of a deleted message, replies only in the same chat).
• Frontend 109 passed; new test_chat_edit_delete.py (reply with a quote, edit and delete your own, others' edits and deletions appear live, a Teacher deletes a Student's message and the quote says so).

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