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/smtpd/smtpd.go

@@ -29,15 +29,20 @@ type Stats struct {
 	LastMailAt atomic.Value // time.Time of last accepted mail
 }
 
+// IndexCallback is called after a mail is successfully stored, with the raw
+// bytes and the storage ID. Used to submit to the async index worker.
+type IndexCallback func(raw []byte, id string)
+
 // Daemon is the embedded receive-only SMTP server.
 type Daemon struct {
-	cfg     config.SMTPConfig
-	store   *storage.Store
-	logger  *slog.Logger
-	stats   Stats
-	server  *smtp.Server
-	mu      sync.Mutex
-	running bool
+	cfg           config.SMTPConfig
+	store         *storage.Store
+	logger        *slog.Logger
+	stats         Stats
+	server        *smtp.Server
+	mu            sync.Mutex
+	running       bool
+	indexCallback IndexCallback
 }
 
 // New creates a new SMTP Daemon. Call Start() to begin accepting connections.
@@ -51,6 +56,11 @@ func New(cfg config.SMTPConfig, store *storage.Store, logger *slog.Logger) *Daem
 	return d
 }
 
+// SetIndexCallback sets the function called after each successfully stored mail.
+func (d *Daemon) SetIndexCallback(cb IndexCallback) {
+	d.indexCallback = cb
+}
+
 // Start launches the SMTP daemon in a background goroutine.
 // It returns immediately; use Stop() for graceful shutdown.
 func (d *Daemon) Start() error {
@@ -237,6 +247,12 @@ func (s *session) Data(r io.Reader) error {
 	s.daemon.stats.LastMailAt.Store(time.Now())
 	s.daemon.logger.Info("SMTP: mail stored", "id", id, "from", s.from,
 		"rcpts", strings.Join(s.rcpts, ","), "bytes", len(raw), "ip", s.remoteIP)
+
+	// Submit to async index worker if callback is set
+	if s.daemon.indexCallback != nil {
+		s.daemon.indexCallback(raw, id)
+	}
+
 	return nil
 }