feat(agent-server): add reversible conversation archiving

[maxgiraldo]

HUMAN:

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AGENT:

This pull request was created by an AI agent (OpenHands) on behalf of Max Giraldo.

Why

Agent Canvas currently stores archive state only in browser storage. The Agent Server needs a reversible, persisted archive contract so state follows the backend and remains available across clients.

Summary

• Add persisted archived_at metadata plus idempotent archive and unarchive endpoints.
• Default search and count to active conversations, with ARCHIVED and ALL filters.
• Mirror the contract in @openhands/typescript-client and cover persistence, filtering, reversibility, history preservation, and deletion separation.

Issue Number

No linked issue.

How to Test

• Backend affected suite: 206 tests passed.
• Archive integration test: 2 tests passed, including active/archived/all search and count behavior.
• TypeScript client: lint and build passed; 310 coverage tests passed; formatting check passed.
• Local Agent Canvas stack was restarted against this branch and its live OpenAPI exposed archive, unarchive, and archive_filter.
• Mandatory final pair review completed with zero suggestions.

Video/Screenshots

Not applicable; this PR provides backend and typed-client contracts.

Design Doc

Not included.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

This is the backend and TypeScript-client dependency for the coordinated Agent Canvas server-backed archiving branch. The frontend PR must wait for a released client version containing this contract.