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/scanner.go

@@ -62,7 +62,7 @@ func (h *Handler) ScanLibrary(c *echo.Context) error {
 		}
 
 		// Fetch library folders from database
-		libraryFolders, err := h.db.GetLibraryFolders(c.Request().Context(), pgtype.UUID{Bytes: [16]byte(libraryUUID), Valid: true})
+		libraryFolders, err := h.db.GetLibraryFolders(c.Request().Context(), pgtype.UUID{Bytes: libraryUUID, Valid: true})
 		if err != nil {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "library not found or has no folders"})
 		}
@@ -261,7 +261,7 @@ func (h *Handler) StartWatchMode(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library id"})
 	}
 
-	if err := h.StartWatchModeForLibrary(c.Request().Context(), pgtype.UUID{Bytes: [16]byte(libraryID), Valid: true}, pgtype.UUID{Bytes: [16]byte(userUUID), Valid: true}); err != nil {
+	if err := h.StartWatchModeForLibrary(c.Request().Context(), pgtype.UUID{Bytes: libraryID, Valid: true}, pgtype.UUID{Bytes: userUUID, Valid: true}); err != nil {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}
 
@@ -289,7 +289,7 @@ func (h *Handler) StopWatchMode(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library id"})
 	}
 
-	if err := h.StopWatchModeForLibrary(pgtype.UUID{Bytes: [16]byte(libraryID), Valid: true}); err != nil {
+	if err := h.StopWatchModeForLibrary(pgtype.UUID{Bytes: libraryID, Valid: true}); err != nil {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}