feat(PROJ-45): IMAP Per-Folder UID-Tracking + UIDVALIDITY-Check

- FolderState: GetFolderState, UpsertFolderState, ListFolderStates, DecideResync
- syncFolder nutzt per-folder UID-Tracking statt globalem highest_uid
- UIDVALIDITY-Check loest automatisch Full-Resync aus
- imap_folder_state Tabelle in initSchema (CREATE TABLE IF NOT EXISTS)
- SetAuditLogger in main.go verdrahtet

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

cmd/archivmail/main.go

@@ -421,6 +421,7 @@ func main() {
 		})
 	}
 	imapSched := imapstore.NewScheduler(imapSt, imapImp, logger)
+	imapSched.SetAuditLogger(audlog) // PROJ-45: tenant-visible UIDVALIDITY-reset audit entries
 	imapSched.Start()
 	defer imapSched.Stop()
 	srv.SetImap(imapSt, imapImp, imapSched)