feat(sessions): opt-in auto-archive of stale sessions + durable pin flag

New sessions.auto_archive / auto_archive_days config: soft-hide (never
delete) sessions with no activity for N days, aging on last activity
rather than creation so an old-but-active chat is spared. Sweeps are
throttled through state_meta and fire from CLI startup, gateway startup
+ hourly housekeeping, and the serve/dashboard backend (opportunistic
on session list + an hourly lifespan ticker), so every surface honours
one setting.

A new pinned column (declaratively migrated) exempts sessions from the
sweep; PATCH /api/sessions/{id} accepts pinned and flips the whole
compression lineage as a unit, mirroring set_session_archived.

cli.py

@@ -1972,6 +1972,16 @@ def _run_state_db_auto_maintenance(session_db) -> None:
             logger.debug("Orphan compression finalize skipped: %s", _finalize_exc)
 
         cfg = (_load_full_config().get("sessions") or {})
+
+        # Auto-archive (soft-hide stale sessions) is independent of the
+        # destructive auto_prune sweep — run it first, before prune's early
+        # return, so enabling one doesn't require the other.
+        if cfg.get("auto_archive", False):
+            session_db.maybe_auto_archive(
+                idle_days=float(cfg.get("auto_archive_days", 3)),
+                min_interval_hours=int(cfg.get("min_interval_hours", 24)),
+            )
+
         if not cfg.get("auto_prune", False):
             return
         session_db.maybe_auto_prune_and_vacuum(