fix: address review feedback on error handling

- Handle json.Marshal error in deepCopyEvent instead of silently discarding
- Handle json.Unmarshal error instead of silently discarding
- Add explicit error checks with panic (appropriate for internal marshaling errors)
- Document that panics signal impossible conditions for valid Event structs

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

store/memory.go

@@ -24,12 +24,20 @@ func NewInMemoryEventStore() *InMemoryEventStore {
 
 // deepCopyEvent creates a deep copy of an event to ensure immutability.
 // This prevents modifications to the event after it's been persisted.
+// Panics if JSON marshaling/unmarshaling fails, which should never occur
+// for a valid Event structure.
 func deepCopyEvent(event *aether.Event) *aether.Event {
 	// Use JSON marshaling/unmarshaling for a complete deep copy
 	// This ensures all nested structures (maps, slices) are copied
-	data, _ := json.Marshal(event)
+	data, err := json.Marshal(event)
+	if err != nil {
+		panic("failed to marshal event: " + err.Error())
+	}
 	var copy aether.Event
-	json.Unmarshal(data, &copy)
+	err = json.Unmarshal(data, &copy)
+	if err != nil {
+		panic("failed to unmarshal event: " + err.Error())
+	}
 
 	// Preserve empty metadata maps (JSON unmarshal converts empty map to nil)
 	if event.Metadata != nil && len(event.Metadata) == 0 && copy.Metadata == nil {