feat(PROJ-34): Retention-Tab + pro-Mandant Aufbewahrungsfristen

- tenantstore: retention_days Spalte, GetRetentionDays/SetRetentionDays
- storage.Save(): per-tenant retention überschreibt globale config
- API: GET /api/admin/retention, PUT /api/admin/tenant/{id}/retention
- Frontend: RetentionTab mit globaler Policy-Anzeige, Mandanten-Tabelle,
  Bearbeiten-Dialog und Purge-Button (superadmin only)

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

cmd/archivmail/main.go

@@ -177,6 +177,7 @@ func main() {
 	}
 	srv := api.New(apiCfg, mailStore, idx, authMgr, users, audlog, logger)
 	srv.SetVersion(AppVersion, Modules)
+	srv.SetGlobalRetentionDays(cfg.Storage.RetentionDays)
 
 	bind := cfg.API.Bind
 	if bind == "" {

cmd/archivmail/version.go

@@ -12,17 +12,17 @@ const AppVersion = "0.9.1"
 // MAJOR: Interface-Änderungen, Breaking changes innerhalb des Moduls
 // MINOR: Neue Funktionen, Bugfixes, Security-Patches
 var Modules = map[string]string{
-	"storage":     "1.7", // PROJ-33 MailWithUID, GetMailsWithUID, GetMailsByRecipient
+	"storage":     "1.8", // PROJ-34 per-tenant retention lookup in Save()
 	"smtpd":       "1.3", // PROJ-28 FQDN-Fallback für EHLO-Banner
 	"imapserver":  "1.3", // PROJ-28 FQDN-Greeting (RFC 3501)
 	"auth":        "1.3", // JWT, bcrypt cost 12, TOTP
 	"audit":       "1.1", // PostgreSQL append-only, QueryFilter
 	"index":       "1.0", // Xapian-Wrapper, Async-Worker, Tenant-Index
-	"api":         "1.7", // PROJ-33 IMAP-Modus-Einstellungen
+	"api":         "1.8", // PROJ-34 Retention-API + pro-Mandant Endpoints
 	"userstore":   "1.3", // domain_auditor Rolle, bcrypt, LDAP-Auth
 	"imap":        "1.2", // IMAP-Sync, Scheduler, POP3
 	"labelstore":  "1.0", // Labels, Tenant-Isolation
-	"tenantstore": "1.2", // PROJ-33 imap_mode, GetIMAPMode, SetIMAPMode
+	"tenantstore": "1.3", // PROJ-34 retention_days, GetRetentionDays, SetRetentionDays
 	"ldapconfig":  "1.1", // Pro-Mandant LDAP, TLS
 	"mailparser":  "1.1", // RFC-2822, MIME, MessageID-Extraktion
 }