Implement audit logging functionality across services and middleware

- Introduced a new `auditlog` package to handle audit logging for user actions, including creation, updates, deletions, and authentication events.
- Enhanced existing services (customer, user) to log relevant actions using the new audit logger, capturing details such as actor ID, action type, target type, and success status.
- Added middleware to enrich request context with metadata for audit logging, ensuring comprehensive tracking of user interactions.
- Integrated Elasticsearch for persistent storage of audit logs, with fallback to file-only logging if Elasticsearch is unavailable.
- Updated API documentation to include new audit log endpoints for administrative access.

This update significantly improves the system's ability to track and audit user actions, enhancing security and accountability within the application.

pkg/audit/store.go

@@ -0,0 +1,76 @@
+package audit
+
+import "context"
+
+// Document is the persisted audit log shape stored in Elasticsearch.
+type Document struct {
+	ActorID    string            `json:"actor_id"`
+	ActorType  string            `json:"actor_type"`
+	Action     string            `json:"action"`
+	TargetType string            `json:"target_type"`
+	TargetID   string            `json:"target_id"`
+	At         int64             `json:"at"`
+	IP         string            `json:"ip,omitempty"`
+	RequestID  string            `json:"request_id,omitempty"`
+	UserAgent  string            `json:"user_agent,omitempty"`
+	Success    bool              `json:"success"`
+	Metadata   map[string]string `json:"metadata,omitempty"`
+}
+
+// SearchFilter defines query filters for audit log search.
+type SearchFilter struct {
+	ActorID    string
+	ActorType  string
+	Action     string
+	TargetType string
+	TargetID   string
+	From       int64
+	To         int64
+	Limit      int
+	Offset     int
+}
+
+// SearchResult holds paginated audit log search results.
+type SearchResult struct {
+	Items      []Document
+	TotalCount int64
+}
+
+// Store persists and queries audit documents.
+type Store interface {
+	Save(ctx context.Context, doc Document) error
+	Search(ctx context.Context, filter SearchFilter) (*SearchResult, error)
+	Close() error
+}
+
+// noopStore discards audit documents when Elasticsearch is disabled.
+type noopStore struct{}
+
+func (noopStore) Save(context.Context, Document) error { return nil }
+
+func (noopStore) Search(context.Context, SearchFilter) (*SearchResult, error) {
+	return &SearchResult{Items: []Document{}}, nil
+}
+
+func (noopStore) Close() error { return nil }
+
+// NoopStore returns a store that performs no persistence.
+func NoopStore() Store {
+	return noopStore{}
+}
+
+func entryToDocument(entry Entry) Document {
+	return Document{
+		ActorID:    entry.ActorID,
+		ActorType:  entry.ActorType,
+		Action:     entry.Action,
+		TargetType: entry.TargetType,
+		TargetID:   entry.TargetID,
+		At:         entry.At,
+		IP:         entry.IP,
+		RequestID:  entry.RequestID,
+		UserAgent:  entry.UserAgent,
+		Success:    entry.Success,
+		Metadata:   entry.Metadata,
+	}
+}