fix: reap expired sessions with a periodic background sweep

The sessions table only grew — find_active ignored expired rows but nothing
deleted them, so lapsed sessions accumulated indefinitely.

- repo::session::delete_expired: indexed DELETE (sessions_expires_idx) returning
  the reaped count.
- app::build spawns a detached hourly reaper (SESSION_GC_INTERVAL) that calls it,
  independent of crawler/analysis config.

Test: delete_expired_removes_only_lapsed_sessions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.9",
+  "version": "0.128.10",
   "private": true,
   "type": "module",
   "scripts": {