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/tenant_worker.go

@@ -6,9 +6,9 @@ import (
 )
 
 // TenantIndexWorker processes MailDocument indexing requests asynchronously,
-// routing each document to the correct per-tenant Xapian index via TenantIndexManager.
+// routing each document to the correct per-tenant index via TenantIndexer.
 type TenantIndexWorker struct {
-	mgr    *TenantIndexManager
+	mgr    TenantIndexer
 	queue  chan MailDocument
 	done   chan struct{}
 	wg     sync.WaitGroup
@@ -16,7 +16,7 @@ type TenantIndexWorker struct {
 }
 
 // NewTenantWorker creates a new TenantIndexWorker with the given queue capacity.
-func NewTenantWorker(mgr *TenantIndexManager, queueSize int, logger *slog.Logger) *TenantIndexWorker {
+func NewTenantWorker(mgr TenantIndexer, queueSize int, logger *slog.Logger) *TenantIndexWorker {
 	if queueSize <= 0 {
 		queueSize = 1000
 	}