Chat sidebar: lazy Archived + System groups, unbounded conversation feed

[arsenmuk]

Problem

The sidebar silently dropped active chats: GET /api/sessions capped non-starred sessions at 50, and cron sessions (bumped every heartbeat tick) crowded conversations out of those slots. Archived sessions had no UI at all — and no unarchive path existed anywhere in the backend.

Changes

Backend

• GET /api/sessions now serves list_active_sessions(): all non-archived conversation sessions, unbounded; cron/hook sources excluded from the feed. archived_count + system_count ride on the payload (two COUNT(*)s), so collapsed groups show a badge without fetching rows.
• New GET /api/sessions/archived and GET /api/sessions/system — fetched lazily, only when the corresponding sidebar group is expanded.
• New POST /api/sessions/{id}/unarchive (restores to idle, clears archived_at, logs an unarchived event) — inverse of archive.
• PATCH /api/sessions/{id} with starred: true on an archived session unarchives + stars in one write, so the star→project hook fires on a live session.
• Telegram's session picker keeps the old bounded list_sessions — untouched.

Frontend

• New Archived and System groups at the bottom of the sidebar: collapsed by default (not persisted), lazy-fetch on first expand with spinner/empty states, hidden entirely when their count is 0.
• Archived rows use the standard session item with a Star / Rename / Unarchive / Delete menu.
• Date buckets simplified to Last hour / Last 3 hours / Today / This week / Other; the first two are elapsed-time (correct across midnight); empty buckets never render.

Testing

• pytest tests/test_sessions.py — 70/70 (10 new: unarchive lifecycle/events/missing, archived list/count, star-unarchive write, active-feed exclusions, unbounded feed regression, system list/count).
• npm run build (tsc + vite) green.
• Validated live: no /archived / /system request until expand; unarchive returns a session to its date group; starring an archived session lands it in Starred.

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