feat(PROJ-30): Xapian → Manticore Search Migration

- internal/index/manticore.go: ManticoreTenantManager + manticoreIndex (RT-Indizes, CGO-frei)
- internal/index/index.go: TenantIndexer Interface (Xapian + Manticore)
- internal/index/tenant_worker.go: mgr-Typ auf TenantIndexer Interface
- internal/api/server.go: idxMgr auf TenantIndexer Interface
- config/config.go: IndexConfig.ManticoreDSN Feld
- cmd/archivmail/cmd_reindex.go: reindex Subkommando
- cmd/archivmail/main.go: Manticore-Branch + reindex Case
- go.mod: github.com/go-sql-driver/mysql v1.8.1
- update.sh: Manticore auto-install, CGO_ENABLED=0, config.yml migration, auto-reindex

fix(IMAP): TCP-Deadline-Wrapper für steckengebliebene Imports
fix(auth): Email-Claim in JWT für User-Isolation
fix(search): User-Isolation via sess.Email (fail-safe)
fix(ui): Admin-Login Auth-Cache, Logout-Redirect, IMAP-Polling-Resilienz

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

internal/api/server.go

@@ -80,7 +80,7 @@ type Server struct {
 	ldapStore         *ldapcfg.Store
 	tenantStore       *tenantstore.Store
 	tenantLdapStore   *ldapcfg.TenantStore
-	idxMgr            *index.TenantIndexManager
+	idxMgr            index.TenantIndexer
 	appVersion           string
 	moduleVersions       map[string]string
 	globalRetentionDays  int // from storage config (PROJ-34)
@@ -109,7 +109,7 @@ func (s *Server) SetPop3(store *pop3store.Store, importer *pop3store.Importer) {
 }
 
 // SetIndexManager wires the per-tenant index manager into the API server (PROJ-21 Phase 4).
-func (s *Server) SetIndexManager(mgr *index.TenantIndexManager) {
+func (s *Server) SetIndexManager(mgr index.TenantIndexer) {
 	s.idxMgr = mgr
 }