feat(PROJ-9): implement labels backend - DB schema, labelstore, API handlers, Xapian integration

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

cmd/archivmail/main.go

@@ -24,6 +24,7 @@ import (
 	"github.com/archivmail/internal/auth"
 	imapstore "github.com/archivmail/internal/imap"
 	"github.com/archivmail/internal/index"
+	"github.com/archivmail/internal/labelstore"
 	ldapcfg "github.com/archivmail/internal/ldapconfig"
 	pop3store "github.com/archivmail/internal/pop3"
 	"github.com/archivmail/internal/smtpd"
@@ -191,6 +192,15 @@ func main() {
 	srv.SetTenants(tenantSt)
 	srv.SetIndexManager(idxMgr)
 
+	// PROJ-9: Label store
+	labelSt, err := labelstore.New(cfg.Database.DSN())
+	if err != nil {
+		logger.Error("label store init failed", "err", err)
+		os.Exit(1)
+	}
+	defer labelSt.Close()
+	srv.SetLabels(labelSt)
+
 	// Start SMTP daemon with index worker integration
 	if cfg.SMTP.Bind == "" {
 		cfg.SMTP.Bind = fmt.Sprintf(":%d", cfg.Server.SMTPPort)