feat(PROJ-21): Phase 2+3+5+8 Multi-Tenancy + PROJ-2 EML/MBOX Upload

Phase 2a: userstore domain_admin/superadmin Rollen, User.TenantID,
          ListByTenant, UpsertLDAPUser mit tenantID
Phase 2b: storage.Save() mit tenantID *int64, email_refs Tabelle,
          GetTenantForMail, GetAllIDsByTenant, StatsByTenant
Phase 2c: JWT-Claims tenant_id/tenant_slug, Session.TenantID,
          Login Domain-Erkennung via E-Mail-Domain
Phase 3:  tenantMiddleware, Handler-Filterung (Users, Mail, Stats)
Phase 5:  SMTP Domain-Routing via DomainToTenantFunc Callback,
          config smtp.tenant_routing + default_tenant_id
Phase 8:  archivmail migrate-tenants Subkommando
PROJ-2:   Upload-Seite /admin/upload mit DropZone + Progress-Polling

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

internal/imap/importer.go

@@ -23,6 +23,7 @@ type Importer struct {
 	mailStore *storage.Store
 	idx       index.Indexer
 	logger    *slog.Logger
+	TenantID  *int64 // optional tenant assignment for stored mails
 }
 
 // NewImporter creates a new Importer wired to the storage and index backends.
@@ -229,8 +230,9 @@ func (imp *Importer) fetchBatch(ctx context.Context, c *imapclient.Client, uids
 
 // storeAndIndex saves a raw email to storage and indexes it.
 func (imp *Importer) storeAndIndex(raw []byte, log *slog.Logger) error {
+	ctx := context.Background()
 	// Save to file storage (deduplicates by SHA256 automatically)
-	id, err := imp.mailStore.Save(raw, time.Now())
+	id, err := imp.mailStore.Save(ctx, raw, time.Now(), imp.TenantID)
 	if err != nil {
 		return fmt.Errorf("save: %w", err)
 	}