refactor: remove unnecessary type conversions and handle ignored errors across codebase

Remove redundant type conversions that Go 1.26 makes unnecessary or that
were already no-ops:

- uuid.UUID(x.Bytes) → x.Bytes (uuid.UUID is [16]byte, same as pgtype UUID Bytes)
- pgtype.UUID{Bytes: [16]byte(u), Valid: true} → pgtype.UUID{Bytes: u, Valid: true}
- (*time.Time)(&x.Time) → &x.Time
- json.RawMessage(x) → x where x is already []byte
- []byte(stringVal) → stringVal where []byte is expected
- int()/int64()/byte() casts on values already of the target type
- Decompressor(fn) → fn (type is identical)

Handle previously ignored error returns:

- collections.go: check json.Unmarshal error in GetCollection
- conversion_service.go: check fileSize.Scan() error
- app_test.go: check app.Shutdown() error in benchmark

internal/handlers/collections.go

@@ -160,12 +160,12 @@ func (h *CollectionHandler) GetCollections(c *echo.Context) error {
 			continue
 		}
 		response = append(response, CollectionResponse{
-			ID:              uuid.UUID(col.ID.Bytes),
+			ID:              col.ID.Bytes,
 			Name:            col.Name,
 			Description:     textToString(col.Description),
 			Color:           textToString(col.Color),
 			Icon:            textToString(col.Icon),
-			AutoAssignRules: json.RawMessage(col.AutoAssignRules),
+			AutoAssignRules: col.AutoAssignRules,
 			CreatedAt:       col.CreatedAt.Time.String(),
 		})
 	}
@@ -214,7 +214,10 @@ func (h *CollectionHandler) GetCollection(c *echo.Context) error {
 
 	var viewSettings map[string]interface{}
 	if len(collection.ViewSettings) > 0 {
-		json.Unmarshal(collection.ViewSettings, &viewSettings)
+		err := json.Unmarshal(collection.ViewSettings, &viewSettings)
+		if err != nil {
+			return err
+		}
 	}
 
 	return c.JSON(http.StatusOK, map[string]interface{}{
@@ -460,8 +463,8 @@ func (h *CollectionHandler) GetDeviceMappings(c *echo.Context) error {
 	response := make([]MappingResponse, 0, len(mappings))
 	for _, m := range mappings {
 		response = append(response, MappingResponse{
-			ID:              uuid.UUID(m.ID.Bytes),
-			CollectionID:    uuid.UUID(m.CollectionID.Bytes),
+			ID:              m.ID.Bytes,
+			CollectionID:    m.CollectionID.Bytes,
 			CollectionName:  m.CollectionName,
 			DeviceShelfName: textToString(m.DeviceShelfName),
 			SyncDirection:   textToString(m.SyncDirection),
@@ -586,7 +589,7 @@ func (h *CollectionHandler) GetBookCollections(c *echo.Context) error {
 	response := make([]CollectionResponse, 0, len(collections))
 	for _, col := range collections {
 		response = append(response, CollectionResponse{
-			ID:          uuid.UUID(col.ID.Bytes),
+			ID:          col.ID.Bytes,
 			Name:        col.Name,
 			Description: textToString(col.Description),
 			Color:       textToString(col.Color),