feat(PROJ-17): Admin Dashboard Systemauslastung immer anzeigen

- Systemauslastungs-Sektion wird immer gerendert (nicht nur bei Erfolg)
- Fehlermeldung wenn /api/admin/system/stats nicht erreichbar ist
- Feature-Status auf In Review gesetzt

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

internal/index/index.go

@@ -0,0 +1,61 @@
+package index
+
+import (
+	"fmt"
+	"time"
+)
+
+// MailDocument is the indexed representation of a stored email.
+type MailDocument struct {
+	ID            string
+	From          string
+	To            string
+	Subject       string
+	Body          string
+	AttachNames   string
+	HasAttachment bool
+	Date          time.Time
+	Size          int64
+}
+
+// SearchRequest specifies search parameters.
+type SearchRequest struct {
+	Query    string
+	From     string
+	To       string
+	OwnEmail string
+	DateFrom *time.Time
+	DateTo   *time.Time
+	PageSize int
+	Page     int
+}
+
+// Hit is a single search result.
+type Hit struct {
+	ID    string  `json:"id"`
+	Score float64 `json:"score"`
+}
+
+// SearchResult holds paginated search results.
+type SearchResult struct {
+	Total int
+	Hits  []Hit
+}
+
+// Indexer is the interface for full-text email indexing.
+type Indexer interface {
+	IndexSync(doc MailDocument) error
+	Search(req SearchRequest) (*SearchResult, error)
+	Delete(id string) error
+	Close() error
+}
+
+// New creates an Indexer for the specified backend.
+func New(dir string, batchSize int, backend string) (Indexer, error) {
+	switch backend {
+	case "xapian":
+		return newXapian(dir)
+	default:
+		return nil, fmt.Errorf("unknown index backend: %q (supported: xapian)", backend)
+	}
+}