feat(core): restore session archiving

[Brendonovich]

Summary

Restores session archiving to the V2 API. V1 supported archiving through session updates, but V2 only exposed the stored time.archived value and had no way to set or clear it.

• PATCH /api/session/:sessionID accepts archived: boolean. true archives the session, false restores it, and repeating the current state is a no-op.
• GET /api/session accepts an optional archived filter. true returns only archived sessions, false excludes them, and omitting it keeps the current behavior of returning both. Pagination cursors preserve the filter.
• New durable session.archived and session.unarchived events project the archive timestamp. Archiving and unarchiving also update time.updated.
• The Solid data store applies both events to cached session info.
• Session rows now treat only null as unarchived, so an archive timestamp of 0 is preserved.

Testing

• bun run check
• packages/core: bun test test/session-archive.test.ts
• packages/server: bun test test/session-update.test.ts
• packages/schema: bun test test/event-manifest.test.ts