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/index/index.go

@@ -54,6 +54,14 @@ type Indexer interface {
 	Close() error
 }
 
+// TenantIndexer manages per-tenant Indexer instances.
+// Implemented by TenantIndexManager (Xapian) and ManticoreTenantManager.
+type TenantIndexer interface {
+	ForTenant(tenantID *int64) Indexer
+	Global() Indexer
+	Close() error
+}
+
 // New creates an Indexer for the specified backend.
 func New(dir string, batchSize int, backend string) (Indexer, error) {
 	switch backend {