feat(PROJ-49): Verschlüsselungspflicht at-rest sichtbar machen (Healthcheck & Warnung)

storage.loadKey() startet bei fehlendem/unlesbarem/ungültigem Keyfile
weiterhin unverschlüsselt (kein Hard-Fail), aber:
- einmalige WARN-Logzeile beim Start mit konkretem Grund
- neuer Healthcheck-Prüfpunkt "Encryption" in archivmail status
- Dashboard-API liefert encryption.enabled
- README: GoBD-Hinweis zu storage.keyfile

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

cmd/archivmail/cmd_status.go

@@ -45,6 +45,7 @@ func runStatus(args []string) {
 		checkPostgres(cfg),
 		checkManticore(cfg),
 		checkStorage(cfg),
+		checkEncryption(cfg),
 		checkAuditLog(cfg),
 	}
 
@@ -173,6 +174,37 @@ func checkStorage(cfg *config.Config) checkResult {
 	return checkResult{Name: "Storage", OK: ok, Detail: detail}
 }
 
+// checkEncryption reports whether at-rest AES-256-GCM encryption is active
+// (PROJ-49). A configured, readable 32-byte keyfile yields status "enabled";
+// everything else yields "disabled" with a concrete reason. Disabled is NOT a
+// hard error (backwards-compatible, see PROJ-49) — the entry stays OK=true so
+// existing unencrypted installations keep a zero exit code, but the detail
+// makes the GoBD-relevant state clearly visible.
+func checkEncryption(cfg *config.Config) checkResult {
+	keyfile := strings.TrimSpace(cfg.Storage.Keyfile)
+	if keyfile == "" {
+		return checkResult{Name: "Encryption", OK: true,
+			Detail: "disabled — kein storage.keyfile gesetzt, Speicher NICHT AES-256-verschlüsselt"}
+	}
+
+	data, err := os.ReadFile(keyfile)
+	if err != nil {
+		return checkResult{Name: "Encryption", OK: true,
+			Detail: fmt.Sprintf("disabled — Keyfile %s nicht lesbar: %v", keyfile, err)}
+	}
+	raw := strings.TrimSpace(string(data))
+	decoded, derr := base64.StdEncoding.DecodeString(raw)
+	if derr != nil {
+		decoded = []byte(raw)
+	}
+	if len(decoded) != 32 {
+		return checkResult{Name: "Encryption", OK: true,
+			Detail: fmt.Sprintf("disabled — ungültiges Keyfile %s (%d Byte, erwartet 32)", keyfile, len(decoded))}
+	}
+
+	return checkResult{Name: "Encryption", OK: true, Detail: fmt.Sprintf("enabled — AES-256-GCM (%s)", keyfile)}
+}
+
 // checkAuditLog verifies that the append-only audit log file (PROJ-48) exists
 // and is writable. It performs a non-destructive O_APPEND open without writing
 // any bytes, so the immutability of existing content is not affected.

cmd/archivmail/encryption_status.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"encoding/base64"
+	"log/slog"
+	"os"
+	"strings"
+)
+
+// warnEncryptionStatus emits a single, clearly visible WARN log line at startup
+// when the at-rest mail storage is not AES-256-encrypted (PROJ-49).
+//
+// It does not change the (backwards-compatible) behaviour of storage.loadKey():
+// the service starts regardless. It only increases visibility so operators do
+// not silently archive unencrypted mails on a GoBD installation.
+//
+// enabled is the result of (*storage.Store).EncryptionEnabled(); keyfile is the
+// configured path (may be empty). When encryption is disabled the keyfile is
+// inspected to log a concrete reason (missing path, unreadable file, wrong size).
+func warnEncryptionStatus(logger *slog.Logger, keyfile string, enabled bool) {
+	if enabled {
+		logger.Info("Speicherverschlüsselung aktiv (AES-256-GCM at-rest)", "keyfile", keyfile)
+		return
+	}
+
+	switch {
+	case strings.TrimSpace(keyfile) == "":
+		logger.Warn("WARNUNG: Keine Verschlüsselung konfiguriert – E-Mail-Speicher ist NICHT AES-256-verschlüsselt. " +
+			"Für GoBD-konforme produktive Installationen storage.keyfile setzen.")
+	default:
+		data, err := os.ReadFile(keyfile)
+		if err != nil {
+			logger.Warn("WARNUNG: Keyfile nicht lesbar – E-Mail-Speicher ist NICHT AES-256-verschlüsselt",
+				"keyfile", keyfile, "err", err)
+			return
+		}
+		raw := strings.TrimSpace(string(data))
+		decoded, derr := base64.StdEncoding.DecodeString(raw)
+		if derr != nil {
+			decoded = []byte(raw)
+		}
+		logger.Warn("WARNUNG: Ungültiges Keyfile (≠ 32 Byte) – E-Mail-Speicher ist NICHT AES-256-verschlüsselt",
+			"keyfile", keyfile, "bytes", len(decoded), "expected", 32)
+	}
+}

cmd/archivmail/main.go

@@ -134,6 +134,11 @@ func main() {
 	}
 	defer mailStore.Close()
 
+	// PROJ-49: Sichtbarkeit der At-Rest-Verschlüsselung. loadKey() bleibt
+	// abwärtskompatibel (kein Hard-Fail), hier wird der Status nur sichtbar
+	// gemacht. Bei deaktivierter Verschlüsselung einmalige deutliche Warnung.
+	warnEncryptionStatus(logger, cfg.Storage.Keyfile, mailStore.EncryptionEnabled())
+
 	// Index — per-tenant index manager (PROJ-21 Phase 4)
 	indexBackend := cfg.Index.Backend
 	if indexBackend == "" {