Expand sentiment to 10 categories, add DB maintenance job, update README

- Expand LLM sentiment classification from 3 to 10 categories: positive,
  negative, neutral, excited, sarcastic, frustrated, curious, grateful,
  humorous, supportive — with emoji reactions for each
- Add daily DB maintenance job at 03:00 UTC to purge stale caches,
  expired rate limits, old logs, and run SQLite optimize
- Add migrate.sql for upgrading existing databases
- Update README: add !sentiment command, maintenance job, rate limit
  config, in-memory message buffer docs, SDK migration history,
  remove references to previous private repo

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.go

@@ -277,6 +277,12 @@ func setupScheduledJobs(
 	c.AddFunc("@every 30s", func() {
 		plugin.FirePendingReminders(client)
 	})
+
+	// Database maintenance at 03:00 daily
+	c.AddFunc("0 3 * * *", func() {
+		slog.Info("scheduler: running database maintenance")
+		db.RunMaintenance()
+	})
 }
 
 func getRooms() []id.RoomID {