Turn System.GetStatus into a live health/readiness surface backed by
real daemon state: process uptime, accounts_loaded, unread_count,
last_error, outbox_depth, per-account sync state (idle/syncing/error
with last-synced/last-error), and top-level ready/db_healthy.
- proto: extend GetStatusResponse; add AccountSyncState message + SyncState
enum; regenerate the descriptor golden.
- store: add cheap ping (SELECT 1), unread-message count, and pending
outbox-mutation count.
- sync dispatcher: retain each account's last completed outcome and expose
a live per-account sync-status snapshot.
- daemon: populate every field from live state; DB read failures degrade
honestly (reflected via db_healthy), never fabricated.
- cli: render the full health surface in text and JSON.
Refs #364
James Maes