feat(PROJ-5): AES-256-GCM Verschlüsselung, PostgreSQL Metadaten, Async Index Worker

- Storage: AES-256-GCM Verschlüsselung (keyfile, graceful fallback bei fehlendem Key)
- Storage: PostgreSQL emails-Tabelle mit Auto-Migration
- Storage: Save/Delete/Stats/FirstAndLastMail nutzen DB wenn verfügbar
- Index: Async IndexWorker (Go-Channel, Queue 1000, non-blocking Submit)
- SMTP: IndexCallback für async Indexierung nach Mail-Eingang
- main: Backfill beim Start (40 Mails migriert + indexiert)
- Bestehende Mails werden transparent entschlüsselt (Fallback auf Raw)

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

internal/index/worker.go

@@ -0,0 +1,90 @@
+package index
+
+import (
+	"log/slog"
+	"sync"
+)
+
+// IndexWorker processes MailDocument indexing requests asynchronously via a
+// buffered channel. It serialises writes to the underlying Indexer (important
+// for Xapian which only allows one writer at a time).
+type IndexWorker struct {
+	idx    Indexer
+	queue  chan MailDocument
+	done   chan struct{}
+	wg     sync.WaitGroup
+	logger *slog.Logger
+}
+
+// NewWorker creates a new IndexWorker with the given queue capacity.
+func NewWorker(idx Indexer, queueSize int, logger *slog.Logger) *IndexWorker {
+	if queueSize <= 0 {
+		queueSize = 1000
+	}
+	return &IndexWorker{
+		idx:    idx,
+		queue:  make(chan MailDocument, queueSize),
+		done:   make(chan struct{}),
+		logger: logger,
+	}
+}
+
+// Submit enqueues a document for background indexing. If the queue is full,
+// the document is dropped and a warning is logged.
+func (w *IndexWorker) Submit(doc MailDocument) {
+	select {
+	case w.queue <- doc:
+		// queued
+	default:
+		w.logger.Warn("index worker: queue full, dropping document", "id", doc.ID)
+	}
+}
+
+// Start launches the background goroutine that processes the queue.
+func (w *IndexWorker) Start() {
+	w.wg.Add(1)
+	go func() {
+		defer w.wg.Done()
+		w.logger.Info("index worker: started", "queue_size", cap(w.queue))
+		for {
+			select {
+			case doc, ok := <-w.queue:
+				if !ok {
+					// Channel closed, drain complete
+					return
+				}
+				if err := w.idx.IndexSync(doc); err != nil {
+					w.logger.Error("index worker: index failed", "id", doc.ID, "err", err)
+				}
+			case <-w.done:
+				// Drain remaining items in the queue before exiting
+				for {
+					select {
+					case doc, ok := <-w.queue:
+						if !ok {
+							return
+						}
+						if err := w.idx.IndexSync(doc); err != nil {
+							w.logger.Error("index worker: index failed (drain)", "id", doc.ID, "err", err)
+						}
+					default:
+						return
+					}
+				}
+			}
+		}
+	}()
+}
+
+// Stop signals the worker to drain remaining items and stop. It blocks until
+// the worker goroutine has exited.
+func (w *IndexWorker) Stop() {
+	close(w.done)
+	w.wg.Wait()
+	w.logger.Info("index worker: stopped")
+}
+
+// QueueLen returns the current number of items waiting in the queue.
+func (w *IndexWorker) QueueLen() int {
+	return len(w.queue)
+}