feat(conversations): archive and delete conversations

[scotwells]

What

People can now clean up their Patch conversation history.

• Archive hides a conversation from the history list, the resume picker and "continue last". It stays resumable by id and can be unarchived. Sending a new message to an archived conversation brings it back.
• Delete is permanent: the conversation and its messages are removed. Every surface asks before deleting.

Where

• CLI (datumctl assistant and patch): conversations archive | unarchive | delete <id> and conversations list --archived. delete prompts for confirmation, and --yes skips the prompt for scripts.
• Full-screen chat: the resume picker adds ctrl+a to archive, ctrl+d to delete, and an Active/Archived view. /archive archives the conversation you're in.
• Portal: history now comes from the server, so it follows you across browsers. Each chat has archive, unarchive and delete actions.

Access

A new conversation-editor role grants archive and delete. The standard user role includes it, so project members get these actions by default.

Before merging

• Portal typecheck: fails until feat(assistant): archive chats and confirm deletes in the history panel datum-cloud/datum-ui#205 is released (2.10.0) and the plugin's dependency is bumped. The build itself passes.
• Postgres tests: not yet run against a real database. The same test suite passes on the in-memory store and through the full API handler chain.
• End-to-end: not yet tried end to end on a cluster with the Milo authorizer.

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